This option would not be as cheap as changing ring and pinions, but it would give the best of both. You would have the 3.73 for towing and with a .78 overdrive the equivalent to a 2.91 for flat land fuel economy. Rpms would drop from 2000 at 68 MPH to 1540.
I had a Doug Nash/U&S gear overdrive on my old '73 Chevy C30 with a Perkins 354 Turbo Diesel. Too bad US Gear does not make overdrives that bolt up between out transmissions and transfer cases.

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2006 Pearl Green CRD
Magnaflow 2 1/2" Cat Back
KJ Extra Leg Room Brackets, Carter Lift Pump, V6 Airbox, ORM
Fuel cooler, Oil Separator, Progard 7
Gauges EGT Boost Trans Temp Oil Pres, Michelin LXT AT2 245 70 R16
7,000# Draw Tight hitch, PML EX Deep Trans Pan
Centrifuge, SunCoast, Transgo, RAM TCM, InMotion Stage 2
Wife's 99 TDI VW Beetle