2001 Ford Ranger like a dog on a chain

At first it was only happening at speeds around 140km (about 70mph) but now even at 40km it does it. At about 80km/h it will try to shift into a higher gear, then it feels like I have a toe chain thats attatched to my truck and then it gets at the end. Almost like a dog on a chain. And it will kind of kick up and then when I let off the gas it will stop. But at 40km (started last night), it will try to shift, and the front end will go down, like normal, but it will stay like that for at least a second. Also, when it does this, it loses power and speed. Why? Could it be because I need a tune up which I am long overdue for? Or could it be the rear diff (which is my 2nd guess), or option 3 (i hope to god its not this one) an overhaul on the tranni. Please help!