I have a 2x4 06 with the automatic. the thing has so mutch power that i think an after market overdrive could greatly increase milage and the Cummins would still have decint power.
can anyone recomend a good overdrive.

 

Gear Vendors is the only thing easily and readily available on the market that I know of for manuals. They make OD's for autos too. I have 10 forward gears with my splits now.You should have 8 or maybe 7 I'm not certain. Google for them and call..

 

There are also 2-speed rear differentials out of the mid-sized construction and dump trucksat junk yards cheap...but they are really beefy.