Which u-joint? What driveshaft or axle?
If an outer retainer, take a pliers and squeeze together the ears that are in the center. Depending on how much rust there is, you may have to take a screwdriver and destroy the retainer.
Inner C-Clips are as easy as taking a screwdriver and a hammer, catching an edge and working it off.
-Ron
edit: I just re-read the title. Wheel joint, right? Inner C-Clip on joint, follow the second methode.