The problem is the tail gate switch same happenened to me in my 87 also found out conection to window motor faulty so went from the connection behind drivers rear taillight to motor but if that fail identify the wires at dash switch and run new wire to back for window.
My tailgate motor runs but no action. Just a small, kinda thumping sound....either the motor gears are shot or I'm not engaging. Since you have apparently "been there, done that". Motors aren't cheap enough to just buy one because....I Also it wouldn't hurt to spray some silcione in the felt track to help it travel better so less torque on the motor and I would take off the TG skid plate and look inside and see how everything looks and do a little maintenance check for rust, maybe smear some grease on the backing plate teeth that match up to the TG motor...........to help extend the life because it's a PITA to get out due to the rivets.......................:doh0715:
Just a thought ~
Good Luck ~ :thumbup