91 F150 Tranny Problems

My tranny has been acting like it is slipping when I put my foot into it to keep my speed while going up a hill. It only does it when the torque convertor is locked up and when the tranny is in overdrive. It is fine when the torque convertor isn't locked or when I just run drive.
Thanks in advance

You really shouldn't have it in OD when going up a hill, but anyway I imagine that at some point prior to this is worked fine that way...

Check your fluid level in the tranny, check the tranny fluid also for either a burnt smell and/or it turning brownish/black. If that all appears to be fine try changing your tranny filter.

One other thing... fuel delivery problems can sometimes appear to be transmission problems, I almost dropped a tranny for what turned out to be nothing more than a plugged up fuel filter that cost me $3 to replace which solved the problem.