Truck would not go into gear

There are only a few things you can check visually;

Check that the shifteris adjusted correctly as in neautral it should place lever into neautral position on transmission.

Check the shift arm on transmission with shifter disconnected and see if lever goes into first, with engine off, or at least feels like it will. Check for excessive play.
Check for excessive play in shifter.

Other than checking the clutch to ensure it is pushing clutch arm, the rest of the issues are internal.

You could have a worn shift fork that is not pushing the gear over the synchro. This is definatley a suspect if the transmission will shift into other gears.

That is about all you can check without removing the tranmission and inspecting it internally.

When the clutch is pushing on the pressure plate and disengaging it, which is what you need to put it into gear, you will feel the strong springs from the pressure plate as resitance in the pedal. This is true for both a cable, linkage or hydraulic clutch actuation system. If the clutch IS NOT pushing on the pressure plate and disengaging it you will feel very little resistance as the only force that will resist pedal movement, if all resitance from a partially working clutch is disregarded, is the return spring form the pedal. I a lot of cases this spring will not lift the pedal all the way from the floor. If pushed the clutch pedal will stay on the floor as this spring is only meant to keep any weight from the pedal from pushing on the clutch.
So, if you feel something that is about the resitance in the pedal that you have become acustomed to, it is pushing on the clutch and disengaging it. If you notice a change in the resistance of the clutch pedal, there may be a change in the cutch actuation from the pedal to the pressure plate.

Do you have a hydraulic clutch?

I forgot whether it is or not and I do not have service manual access at this moment as I am just checking for responses.

As far as, "the forks", that you are referring to, I think you might have a different idea as to what they are. Shift forks are internal to the transmission and are what the shift arm on the side of the transmission is linked to in the transmission and the shifter is linked to the shift arm. Shift forks actually push the gear cog into gear. For instance if you have a bent fork, it may not be able to push the gear far enough to engage a gear as it is bent away from that direction of travel.
So a, "fork" is not something that you will feel in the pedal. It is something that will keep the transmission from going into a gear. That is why the question it relates to is, "Does the transmission go into all or other gaers and not one gear"?
You can usually get a tranny into a gear if it has not been loaded by the weight of the vehicle. It is not something that has anything to do with the clutch pedal at all.

The shift arm that you see is all there is. The shifter arm connects to the shift arm of the transmission, I hope all the shift arm stuff is not getting confusing because I am thinking I may have confused you along the way as you thought the reference to the shifter as a, "shift arm" or having a shifft arm is not correct.
To make sure;
The shifter motion is transfered on a pivot through the floor board the the attached shift linkage, that is where I think I confused you, which then is attached to the shift arm of the transmission which is directly attached to the internal shift fork. So again, it goes;
Shifter connected to linkage connected to shift arm connected to fork connected to gear cog.
I hope that clears some stuff up as I may have refered to the shift linkage by mistake and/or the part of the shifter beyond the pivot, where the motion of the shifter changes direction, gets confused as a shift arm sometimes.

As far as videos to look at I do not believe we have anything that will be specific enough to look at. You can try YouTube as it will probably have a video of your actual truck and someone working on the shifter. You can find anything on there. The reason we do not, I will double check, have a shifter video is that it can be confusing and gets to be a little farther than most customers will get into.
There is also the fact thaat some shifters attaches to linkage and some to cables and some to a push pull rotating shaft. So, it can get to where a video of your truck would be confusing to someone with a cable shifter, which is a lot like most automatic shifters are like.

As far as a direction to go, let me know the following;

Do you have a hydraulic clutch?
Does the transmission shift into any other gears?
Try this with the transmission not being loaded by the weight of the vehicle, like when you park on a hill and use the transmission in gear to hold the truck in place from rolling, with the engine off and see if that makes a difference.

I cannot tell you much yet. If it shifts into other gears and not into just first, it is probably internal.

Have you checked the clutch pedal freeplay?
This can get to where the pedal is not moving the clutch arm and not disengaging the clutch.
On a hydraulic system this is not adjustable but would end up being an issue with air in the clutch system.

Let me know this and we will at least take it as far as possible into diagnosing it as you feel comfortable.

It sounds like you are saying that it is like you pushed the clutch in and it will not come back oput, like when you start letting up on the clutch pedal to slip it and get it moving. So, it goes into gear but will not move.
That sounds like the pressure plate has failed. One or more of the springs which actually clamp the load have failed and the clutch is stuck in the dis-engaged position and will not clamp onto the clutch disk and thusly will not move the truck.
So, it sounds like the tranny must come out to figure it out.
I am confused by the fact that you have a hydraulic clutch and a clutch cable at the tranmission? Don't you have a slave cylinder that pushes a push rod there? Where does the cable come in?
I thought that you may want to try bleeding the clutch system, but that will only keep the clutch from dis-engaging and you would not be able to get it to realease when you pushed the pedal in. Just like your brakes, there is a bleed screw on the slave cylinder and you use brake fluid and bleed the clutch by pumping the pedal and holding it while another person loosens the bleed screw then tightens it and then the pedal can be let up again. You repeat this until you have bleed all the fluid through and are sure their is not any air. Using a clear tube that fits snugly over the bleed screw with a catch bottle is a good way to keep it clean and see if there is air bubbles in the tubing whcih is air in the line.
However, it sounds like what I first mentioned if I am understanding you right.
Let me know and I will respond ASAP.
I am going to reformat my commputer so my normal response time may be a little ongert than usual.