more then likely you need to countersink the holes in your rotor, whats happening is the knurl part of the 1/2 stud sticks out beyond the wheel flange, so when you tighten the wheel it pushes the stud out just alittle, causing it to do what your talking about. if you jack the car up the wheel will feel loose.
this is common anytime you install 1/2 studs no matter front or rear end, and only some rotors need to be countersunk, some are fine and have no issue.
goodluck
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