What are the symptoms pf the problem? More description of what is happening with the tranny would help most of us give you a better answer. There are detent pins and balls under springs behind plugs in the tower that hold it in gear by selectively engaging machined radiuses in the shift rails that carry the shift forks that hold it in gear and prevent it being shifted in to two gears at once. A broken spring in one of them can cause problems. Also the tip of the stick engages the shift rails in a slot in each one. If there is wear on the end of the stick or in the rail slot it engages sometimes the stick can be forced out of one rail into another. When the end of the stick comes out of the rail that is in gear and slides between the rails and goes to the neutral position, then you are locked in gear and moving the stick will have no affect on getting any shifting done. I have used a welder to build up the stick end and the worn rail slots to stop the problem. At any rate, it sounds like all of it is in the top of the trans which is rather easily removed and replaced.
If you find it is stuck in gear, pull out the stick and look down in the tower. If you see a rail that has its stick tip slot out of alignment with the others, put the stick tip or a pry bar in it and pull it to neutral (inline with the others). You can also look at the stick end to see if it is worn and see the rail slots for wear that lets the stick get past one that is in gear into another that is in neutral. If you find these problems with your tower and are not comfortable with welding it, get another one and put it on.
Regards Martin
