Update:
Today I had my last final exam, and I started working on this truck as soon as I got home.
I sanded and cleaned up all of the ground connections to the body, the ground to the motor (which was very hard to get to) and replaced the battery terminals. I also swapped the battery from my other vehicle in as well.
While replacing my battery terminals with new ones, I removed the last inch of wiring that was attached to the old terminals. The positive connection was perfectly fine, but the negative connection had loads of corrosion underneath the wiring insulation. I was honestly surprised by the amount of corrosion underneath, since it looked just fine from the outside. I removed about 4 inches of wire off of the negative, and after that there was much less corrosion.
After doing all of this, my truck started just fine. It actually started faster than I have ever seen it start before. It never took a long time to start before, maybe about a second or two, but when I started it this time it was instant.
Although removing the most corroded section of wire has solved the problem for now, I have a feeling ill eventually have to replace it at some point. My other vehicle seems perfectly happy starting with the other battery so I dont think that was the problem.
A big thank you to everyone who helped my solve this and especially to whoever said to replace the terminals.
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