Almost cranks but will not quite crank?

Okay, you say it cranks but doesn't crank? So, do you mean it turns over when you turn the key but doesn't start? Or does it start up and then shut off after about 3 seconds? If that is the issue, then the fuel pump likely isn't turning on when it should. That system works normally by the relay turning the pump on for about three seconds to prime the system. Then the engine turns over. If it makes oil pressure and actually starts running the oil pressure switch sends power to the fuel pump and it runs. That also acts as a fuel shut off in the event of the engine stalling or an accident that causes something like a roll-over, so the engine loses oil pressure. As soon as the pressure drops the pump shuts off. There are a couple common failures. The pumps and connectors and the oil pressure sender. A simple way to test is to run a jumper wire to the prime connector which is a red wire that is normally wrapped in the harness near the relay. Or if you go to the ALDL connector under the dash and find terminal G and connect 12 volts there, both bypass the relay (relay must be installed though) and oil pressure switch and powers the pump. So, if you can attach power there and the truck runs you know the pump is okay and the wiring from the relay to the pump is good. If it primes the system and tries to start you know the relay is switching and the ECM is turning it on so that wiring is okay. That leaves only the oil pressure switch and the wiring for it. To test it the easiest way you just buy the replacement unit and install it. It is under the distributor at the rear of the engine. The old one unscrews and the new one screws in be careful though, it is a tapered thread and it's very easy to accidently tighten it too far and crack the adapter. I normally apply a couple wraps of Teflon tape and then screw it in just tight. There is no tech spec on that just until it doesn't leak. Now if it runs you should be good.

Did you try bypassing the power to the pump yet? If it runs that way then there is a wiring issue.

You just need a chunk of wire to connect between battery power and the prime wire or the G terminal in the ALDL and see if the pump powers on and stays on and the engine runs. If it still doesn't start and run, then we need to continue.

Anyplace with battery positive, right at the battery works.

Okay, if the jumper didn't work and the fuel pump was running it might be the ignition dropping out. The jumper was to discover if it was the common issue with the fuel system. Sounds like it isn't that. That means we have to discover if it is losing spark. For that a timing light or an inline spark tester would be handy.

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

Okay, so as a recap. You get in, turn the key and the engine turns over, starts and runs for a couple seconds then shuts off? Or does it turn over and not start at all?

It's possible but not likely. I would get a timing light and test it.