I agree, its hard to check a u-joint on the car. I just did my hanger bearing and one week later I thought it was falling off. It was the rear u-joint which I looked at (seemed OK) but didn't remove from the car while doing the bearing.

When I did my bearing (88 765) I removed the front driveshaft section, cut and hacked away the bearing housing, knocked the seals and ball carrier out of the bearing, pushed all the balls to one side and removed the outer race. Now I could carefully tap an old chisel around and between the inner race and shaft collar, once I got it moved 1/4" I could easily tap it off (wd-40). I cleaned up the shaft with 240 emery and the new one tapped right on (only tap the inner race very carefully) tap = small hammer and caution
Good luck