put in 8 3/4, now have vibration

Update: I put in a new u joint with part#6301 which seemed to be a better fit. AutoZone guys weren't real sure about which u-joint to use since I've swapped out my entire drivetrain (I dont even know what year make and model my rear end came out of).

I still have the vibration, seems the same. I think I must have been hallucinating yesterday when I thought it was better for a minute.

Can anybody tell me the most common part number for u-joints used in 8 3/4 rear end? I'm still concerned because the one I put in has a slight amount of play (very slight, not measurable) and I remember in the past that the c-clips were usually a very tight fit into the yoke. Plus - the cups of the u-joint are recessed into the yoke (like the u-joint is too short?) not extended out.

Thanks for your help and any other suggestions!