1994 Tranny problems suggestions needed!

I have a 1994 3.3L Voyager. Started overheating, I changed the thermostat,still hot,then changed the radiator fluid, and then took out the thermostat. The van would be fine and run all day long. If stopped in traffic or a slow idle it would start to heat up, after a couple months of this, it was found out that the radiator fan was bad, replaced it with new, reinstalled the thermostat. No overheating problems now, but tranny drops into "limp" mode now. It was suggested that the tranny be flushed and filter changed. Done at the shop before I discovered this forum. It will run a few miles up the road and shift fine, all gears and after about 10 minutes, it'll drop down and it still doesn't shift outta 2nd and am looking at replacement. Any suggestions would be very helpful. Is it possible that with the radiator running hot it "cooked" the fluid and there by damaged the internals of the tranny? I replaced the crank sensor, the coil pak as well as the O2 sensor with no changes in the tranny.Someone suggested the TCM. Is this the unit mounted on the firewall? Please help! Thanks