when tightening the rear shock, the bottom bolt keeps spinning . How do I fix this. Do I replace the bolt or what?

 

The shock should have a hex on the end of the stud that the nut is tightened on. Use the proper socket or vice-grips to hold that hex to keep it from spinning while you tighten the nut. The alternative is have someone hold the shock shroud from under the car while you tighten the nut.