I have changed many many u joints in my day and have never seen one that was 'tightenable' ever. They are replacement items and are usually not repairable.
Not to expensive either. 15-30 bucks a piece. If this is a driveshaft (two drive shafts on a wrangler. Two u-joints on front shaft, three on rear i believe), then the replacement is rather easy. Mark drive shaft and yoke on both ends, remove nuts on u-bolts holding ujoint end caps and with a soft face hammer, 'persuade' drive shaft out. Ujoints should be pressed in and out, but with a bench vise and a 5lb mallet they are easy enough to remove, after the c clips have been excised (remember that you will NOT need the removed u-joint after repair). Installing them is also possible with a stout c clamp and the correctly sized impact socket as a punch/drift. Assembly reverse of removal
If your talking about front axle u-joints, that is a big job and requires some time, as the entire axle end (brakes, calipers, rotor, hub needs to be disassembled down to the spindle. The front axle removed and the ujoint pressed in out. It is doable by a shade tree back yard mechanic, but is best watched once or twice before attempting yourself.
I'd be happy to answer any other questions you have about the process here.
-bear
Forgot to add. Some u-joints come with a vein which runs through the cast part of the joint and a zirc nipple for greasing the joint. THESE U-JOINTS are weaker then non-greasable u-joints as they have an inherent weakness in that grease vein. I would recommend going with solid non-greasable u-joints as they are stronger, last longer, and are cheaper.