my trans (c6) got stuck in reverse--linkage was ok-- pulled pan and found a loose nut on the inside end of the shift control shaft allowing it to disengage the shift control piston(?) Normal tools won't fit up in there to tighten it. Is there a special tool? What is supposed to keep the nut from coming loose again when I do tighten it?

 

I always install the nut (with a box wrench) before installing the valve body. Perhaps you will need to drop the valve body to gain clearance and be able to get over the kickdown lever. If I remeber right, the C-6 uses a slotted nut - the slots allow the nut to grip the shaft's threads when tightened. If not tightened properly, the nut will not stay put. Art