THis problem can also happen on both 3L and 2.6 Manual trannys as they have the selector rail thru the t\case if you have an auto tranny and have this problem your case could be worn out or like what whitelespaul said. Now that i think of what whitelespaul said if you have a body lift it is very possible that the body is now coming in contact with the shifter. and yes it is internal open your manual I believe it will have a description of how to separate the case from the tranny and it will talk about a pin.

89 2 dr turbo diesel
89/88 "Backwoods sas"