Green&Yellow

I had problems with my 94GC not shifting into overdrive a while back, then the trani started slipping real bad. The intermediate shaft and a plate sheared off, probably due to not getting enough fluid in that area. Anyway, a friend and I rebuilt the trani and got it all back together(major pain in the butt) and in but I am still having the overdrive problem. I disconnected one of the hoses while it was running to make sure that the fluid was being circulated properly and it was pumping away. I feel confident that we installed everything correctly. Anyone have any idea why the overdrive issue? I have heard someone say it was their catalyitc converter and another that it was their thermostat. To me, that doesn't seem relevant to the transmisson. I think those problems were most likely keeping the engine from functioning properly, and not shifting into overdrive was a result of the engine, not the other way around. Anyone have any ideas?