GU6 is the RPO code for the 3.42 axle gear ratio and does not have any connection to the transfer case offered in the vehicle.

The transfer case will either be an NP231 (manual shift, two speed - RPO code NP2) or NP233 (electric shift, two speed - RPO code NP1).

The 1st gens were never equipped with the NV236 (active, electric shift, two speed - RPO code NP8) or the NV136 (active, single speed, AWD - RPO code NP4) transfer cases, but some of the later ones may have come with the BW4472 (single speed, AWD - RPO code NP3).

A bit more info on the truck would certainly help clear up the confusion.