There is an adjustment. From memory you remove the two piece cowl from around the steering column (the one just below the instrument cluster). You should then see where the end of the little shift indicator cable attaches to the shift linkage. You will also see where the outer sleive of it is held in place by a sindle screw. If you slacken that screw you can move the outer sleive to adjust the pointer. When you have it set tighten screw.
Sounds like the clamp is probably broken off, as they tend to do with age. Mine did it on the '86 Bird. I fixed it for a while with zip ties pulled really tight around the pieces of the clamp. I later replaced the cluster (to which the PRNDL cable and clamp is attached, which permanently (until the clamp breaks again) fixed the problem.
the loop of the cable hooks to the column-it should have a plastic adjuster on it-should be behind lower steering column shroud-1986-1990-it sounds like it's out of adjustment or someone has spilled a lot of sodas on the column-sticking!!!
Under the cowl by shifter remove the two screws and the cover comes off. The screw for the adjustment is on top of a white peice and is a 7 MM screw. Loosen to adjust. If not replace it. I got mine from LMCTruck.com