94 3800 Lesabre torque conv lock-up or bad sensor?

Definately check the TCC module in the transmission. It controls the torque converter and has been found to be in adequate for this vehicle. It actually requires a larger solenoid than what it came with. AAMCO can change it out by boring a bigger hole and installing the right solenoid for $300. Most transmission rebuilds do not require this part to be changed out. If you decide to change out the old one with the same solenoid any shop can do it for about $100. The part is only $20 dollars at Auto Zone and can be done yourself if you have the time.