Unless the vanes in your TC are shot, it won't result in slipping. If the vanes in your TC ARE shot, you should have chunks of debris in your pan as well as a rotating eccentric mass (as well as huge vibrations and noise at all speeds). Also, I think the TC locks anyway in higher gears (I could be wrong on this so don't quote me).

Bottom line, I would be stunned if you have a damaged TC resulting in slipping. You most likely have worn clutch packs. Try a bottle of Lucas oil (or something like that) and see if you can limp it along until you get it rebuilt.