* 4L80E four-speed automatic: This was a very common heavy-duty automatic transmission in these trucks.
* Allison 1000 series automatic: This is also likely, especially in higher trim levels or with heavier-duty packages. Several variants of the 1000 series existed.
To know for sure, you'd need to check the vehicle's specifications (either the sticker on the door jamb or the owner's manual) or look at the transmission itself.
