95 Riviera not shifting

The other day my dad's 95 riviera started acting up, The tracton control light has been on for some time now, ECM says left rear wheel sensor, installed new hub from GM, same problem. GM tech said it could be wireing. So we just drove it with no other issues until the other day. backing out of the garage everything was fine. when we started up the road is when the problem started. the car would hardly pull itself, If you get into the gas it was like the trans kicked into neutral with a klunk. The car has 53K on it and it's very clean. We did install a new GM longblock about 1K ago due to oil pump pressure relief stuck and caused spun bearings. Anyway we took it to the garage. the put the computer on and the alt was putting out 18 volts, installed new alt still 18 volts. If you unhook the alt or just run the heater on high to pull down the voltage a little the car runs and drives fine, and the trans shifts great. any ideas anyone? Change ECM? any help is greatly appreciated.

More than likely, the PCM took a dump, however, the T/C problem could be a different issue.