1988 Mustang 5.0 transmission lock up solenoid?

What lock-up solenoid? Are you referring to a lock-up solenoid for the torque converter? If so, it doesn't exist in this particular car, nor any early 80s to early 90s Ford products equipped with the AOD transmissions like the one referenced in your question. Those particular transmissions were unique, in that the transmission's operating characteristics utilized a 100% direct mechanical connection to the engine crankshaft, ELIMINATING the torque converter when it shifted into overdrive. This particular transmission achieves this through the use of TWO input shafts, instead of one, as opposed to having a lock-up clutch inside the torque converter. With that said, there is no torque converter lock-up solenoid in this particular transmission. If this is not what you are asking about, disregard this post, and add further details to your question, or post a new one.