They know that only '98+ cars had the Theftlock, but ONLY the HU is different, the car data bus is the same. So you can install a '96-'97 (non-theftlock) HU in a '98+ car no programming required or you can install a '98+ HU in a '96-'97 car just like you would in a '98+ car (with the VIN clear done).
OK too much reading just print this and show to the technician:
When connecting the TechII to a '96-'97 car to clear the VIN from the HU build the car as if it was a '98+ car (use Eldorado or DeVille) either by manually selecting the year, model OR you can use a VIN of a '98+ vehicle. Do NOT use the VIN of the vehicle you connecting to!
The clear VIN menu should be now available when selecting the HU (IRC).
Proceed as if you would normally do on a '98+ vehicle.
There is one catch with the HU you have, it is indeed a ‘98+ base model but sure hope is not from a Catera as it won’t work, it has to be DeVille/Eldorado/Seville ONLY.