If the center of the ujoint moves seperate of the 4 caps, it's bad

 

So it doesn't make sound if you're sitting still? Or does it make the sound if you are sitting still but have the transmission in gear? If the sound is there while the vehicle is not moving but is in gear, and only when it's in gear, then it might be a transmission problem.
Can you be more clear about when it makes the noise?

 

it makes the noise when it's in gear and I'm givin it gas. when it's in gear and sitting still, it's fine.

 

Okay, so this is a movement thing. (and can we assume that you're in 2-wheel drive?) Next thing I would try is to block the rear axle up off the ground (and put a block around the front tires so the Jeep doesn't move) and see if you still hear the sound and can locate where it's coming from (only do this in 2-wheel drive).

 

That sounds like a sticking/collapsed lifter to me. If it was a u-joint, in theory, there would likely be a wobble accompanying the "tick"

 

I had the same problem, it was actually my center cap that came loose and was clicking when the wheel spun.