Thanks for all of your responses. I do know that the rear derailleur and tensioner are one in the same.
Something like this is what I had in mind:
Originally Posted by
xenologer
a very low budget solution is to simply tighten its range limiter screws (marked H and L) so that it is imobilized on a middle range gear. This will allow you to use it purely as a chain tensioner and make the bike a 3 speed via the front shifter only.
I looked on Park Tools a little and tried to learn about these adjustments, I may try it out this afternoon.
As many of you mentioned, I don't want to alter the drive train too much, I may want those gears later. The shifter itself seems to be the problem, it's like it doesn't have a grip on the cable to pull it, so it can only shift in the other direction. I'm a little nervous to tear the shifter apart and examine it, but I guess I can't make it any worse than it already is.
Thanks again for your help. Any other suggestions?
What else have people done with their 90's GT mountain bikes?