If you have a reverse lock-out that's usually where the problem arises. The roll pin vibrates and eventually gets loose; at least that's what happened to mine. I just slightly drilled through to accommodate the next larger roll pin. That firmed it up..end of problem.
If you are talking about a manual transmission shifter on the floor - There are isolator bushings that go between the two bolts that hold the shifter on and yours are shot. You will also feel a lot of play in the shifter. Now how do you think I know this?