* ZF S5-43 - This was the standard transmission for the 1988 model year. It was a 5-speed manual transmission.
* Ford C6 - This was an optional automatic transmission, although it was less common in the 7.3L diesel trucks.
To be absolutely sure what transmission you have, you'll need to:
1. Check the VIN: The VIN (Vehicle Identification Number) can be used to look up the exact specifications of your truck.
2. Inspect the transmission: Look at the transmission itself. The ZF S5-43 has a specific pattern on the shifter, and the Ford C6 is a larger, more prominent unit.
Let me know if you have the VIN, and I can help you determine the transmission!