I have a 99 silverado lt with a 5.3 4l60e and autotrac, I have a 6.0 I want to install with one problem ,what trans to put behind it . I have three 4l80e's a 96,99,and an 03 I'm not sure which one to use they are all for 2wd so I would need to convert for autotrac need adapter from transmission to transfer case any help would be greatly appreciated thanks

 

I would use the 96 since it's already got both Speed Sensors and the Reluctor Wheel internally for the Rear Sensor.

 

All 4L80Es use 2 speed sensors, and all 2wds will have the rear reluctor wheel internally.

That being said, the 96 will be easier to swap in (due to the cooler line location and lack of needing a special fitting on the rear) and have a stronger OD roller clutch. If you are sending it to a reputable builder, it shouldn't matter which core (as long as you don't plan on doing WOT shifts into OD).