First thing to do is check and see if it has fluid. It won't move if it doesnt have fluid. If no fluid, see where its leaking. Could have a punctured hose or oil cooler and spit the fluid out. Fix it, fill it back up, and your good.

If it does have fluid, what color is it? Should be red or close to it. If it is brown or black, thats bad, especially if its got metal particles in it. Time for a rebuild.

But if the fluid is clean and clear, you might want to check the linkage. Maybe it's broke. If so, try getting under it and shifting it manually while someone else is in the truck.

Personally, I'd go to the junkyard and get a used tranny for $300 or less. If you want a top quality rebuilt unit, check out www.marlincrawler.com (he might only do manual trannies though)