Universal joints are used on most 4-wheel and rear wheel drive cars to transfer drive power from the transmission through the driveshaft to the differential. Primarily, U-joints should be inspected frequently, not just at the 100,000 mile mark. The main component of maintaining universal joints is to make sure that they are well lubricated and are functioning properly. However, a good number of u-joint assemblies are factory sealed and thus cannot be lubricated. If they are damaged or worn then it may be necessary to replace them.
Inspection of the U-joint should include checking for leaking lubricant, rust, or excess wear or play which could manifest itself in a number of ways. This includes: 1) Squeaking that can be heard from under the car, usually when traveling less than 5mph, 2) A clunking noise that occurs when accelerating from a stop, or 3) High-frequency vibrations while driving.