What is the name of automatic transmission in a 1988 Dodge D100?

The 1988 Dodge D100 could have come with a few different automatic transmissions, depending on the engine and options. There's no single name. The most likely candidates are:

* Torqueflite 3-speed (727): This was a very common automatic transmission in Dodge trucks of that era.

* Torqueflite 4-speed (A727, or sometimes referred to as a 4-speed 727): A less common but possible option, especially with larger engines.

You'll need to check the vehicle's documentation (owner's manual, build sheet if available) or look at the transmission itself for a casting number to identify the specific transmission.