That's a tuff one Moze. I'd say the three most important mods to get a truck haulin ass is:
higher stall converter
cam
tune
The truck needs to launch, the engine needs to breathe and the tune brings it all together and increases hp.
Pick one of those, not the tune, and you'll gain some hp. If the budget isn't enough, I'd save up a bit and get a lot more for your money.

If you need something installed to ease the pain, lol, I'd get a shift kit.

hope this helps,
allen