Before you tear into the transmission make sure it's not the sensors acting up and telling the TCC to lock an unlock. Unplugging the trans plug will cause the TCC to never lock. It will also not tell the ECM that you're in 3rd or 4th gear, so don't floor it while unplugged.
If the brake pedal switch is right at the ragged edge of being compressed, the slightest pump can cause it to unlock and relock.
An easy test is to jump out the TCC on the ALDL connector to force the TCC to lock at cruise. If you pop out the paper clip and it immediality goes to bucking, odds are you found your problem.