U-joints are one of those things that can go bad regardless of mileage. They do tend to go bad if things were loose at the yokes where they reside. One easy way of checking the condition of the U-joints is by crawling under the Jeep and grabbing hold of the drive shaft (with the vehicle in gear/park and the brakes set) and twisting it back and forth. The shaft may move a touch but there should be no play in the shaft at all. If there is and nothing else is moving then the joint(s) is/are bad and in need of replacing. Remember to replace all of them while you're under there!
Replacing the u-joints is actually a pretty easy job if you have a press, a vise or even a hammer and a couple of sockets. Head over to Stu Olson's site
for a decent review.