1) is there an overdrive unit that will work with my '05 CTD 4x4 automatic?
2) does it install between transmission and transfer case or for rear axle only?
3) will it help (much)to improve fuel economy at highway speeds running empty? I am thinking that dropping the RPMs 300 - 500 at 65 - 75 mph will help...
4) will it be an issue pulling a 11,000# 5th wheel? (not in overdrive)
Yes it does but there are units that you can buy that bolt on after the tranny and transfer case if it is 4x4 that you can engage at the push of the button that gives you an overdrive in every gear. Helps to give better gas mileage and just offers more gears. Not sure about one for a 4x4 because of the transfer case hopefully someone does becasue i'm not sure
Yes there are units for our trucks. Gear Vendors is one, and US Gear is the other. One of them goes after the transfer case, and then only enguages in 2 wheel drive. The units are rated at very high HP and toque limits as well as towing capacity. They are both very expensive and it is doubtfull that you would see a R.O.I. unless you drive alot of miles and keep the truck a long time.