I have the US Gear overdrive in my 2001 ETH 4x4 and it's great. It fits between the transfer case and the transmission, so it works in or out of 4wd.
You can also use it with an exhaust brake, unlike the GV, which I've heard you'll burn it up if you try to slow down against it.
As for RPM drop in 6th, it's almost 500 RPM difference. 2000 RPM vs 2500 RPM at about 75-80 mph.
Rob
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2001. 5 RAM 3500 QC 4x4 ETH/DEE Silver/Agate, 4. 10, USGear OD, Rickson 19. 5 Wheels, DD EZ, DD3 Injectors, SPA Guages, Psychotty, Firestone Airbags, Hellwig Rear Swaybar, Torklift Superhitch & Camper Tiedowns, Mag-Hytec Diff Cover, DD 4" Exhaust, US Gear Exhaust Brake, etc. On order: Southbend Clutch, HX-40