if your gears are changing automatically, but not till revs are up, then it's the shift solenoids inside the trans. they're relatively cheap and very easy to change. you probably seen them when you took the trans pan off.

if they won't change automatically at all, and you must start off in 1st and manually change gears yourself, then i would replace the TPS. you can cut cost and grab one from a parts jeep/scrap yard.

if all else fails, replace transmission.