put in 8 3/4, now have vibration

Update: Under my car last night messing with the u-joint (the C-clips were on) and discovered it has some side to side play of about 1//32" or 1/64" because apparently the c-clips are not fitting snug into the pinion/yoke where it seats.

I slid it tight to one side then cranked down the little hold down bolts (not too tight because I've broken one of those before). WHen I was driving my car to work on my 10 mile freeway drive this morning I was amazed at how smooth it was.

Then suddenly it started vibrating again, seemingly worse then before. I assume the u-joint got loose and started moving (vibrating) in the yoke/pinion.

I dont know why the u-joint doesnt fit tight when seated. The c-clips should be scraping the edges when seating into the yoke, but such is not the case. I'm goint to buy a new u-joint and hope it fits snug...