They are not for u-joint clearance. They are shims to get the proper tension and seal for the ball housing. Some units need more than 1 shim to get the tension correct so they give you a bunch. If you are getting clunking on deceleration, try looking very closely at the u-joint assembly and the front bushing in the torque tube. When accelerating there is constant pressure on the parts, upon deceleration everything can get loose and sloppy. Look also inside the bell for any marks or scrapes where something is rubbing or hitting.
