What auto transmission is in the 1998 Chevy 3500 4x4 with 6.5 diesel?

The 1998 Chevy 3500 4x4 with the 6.5L diesel engine likely has the Allison 1000 automatic transmission. While there's a small chance of a different transmission in exceptionally rare circumstances (custom builds or very unusual options), the Allison 1000 was the standard heavy-duty automatic for that truck and engine combination.