What overdrive unit for my diesel?

[71-LS6]

Now that the reality of $3/gal, soon to be $4/gal. diesel has set in, I want to install an aftermarket overdrive unit on my diesel. I've got an 05' Dodge 4wd 6spd Cummins diesel dually. I drive everywhere, sometimes towing as much as 15,000 lbs of equipment or my race trailer w/ car.

Who makes the most bulletproof unit? There is no reason that I can't at least drop 500 rpms on level ground at hiway speeds, and when it's empty, I want even more. Has anybody had much experience with the units that bolt on to the tail housing?