[valiant67]

Surely, there must be a simple way to reset the code in the TCM!

Disconnecting the battery does that (at least with the original computer, replacement computers have permanent memory that is not reset).
Assuming the battery disconnect resets the computer but the transmission still remains in limp mode, the fault is a hard code as mentioned above. Some codes will immediately put the transmission into limp mode, others only after they occur a certain number of times during car operation. You really need the diagnostic codes from the transmission to diagnose it.

 

[valiant67]

1988 would not have a TCM (as it used the old 3 speed auto which did not need one and had no solenoid pack).
But the posters above are correct, there will be one there for 1989 in the New Yorker.