2200 RPM at 75 MPH with 3.08s and 35" tires is not bad at all, stock 32" tall tires would require 2400 RPM which is still not terrible. My 7.3 F-450 with 4.88s and 33" tall tires runs at 2700 RPM in overdrive to get to 75 MPH. Starting out with a 4.47:1 effective first gear and 3.73:1 rear gears is about like me starting out in the second actual gear in the ZF6 (the one marked "1" on the ****.) I can do that if the truck is empty and on a level or downhill grade but up a hill or loaded, forget about it, it's in granny low to not abuse the clutch. Even in granny low I have to slip the clutch a bit to not stall out taking off up a hill from a stop with a good load behind me. Granny low in my truck would be almost identical to that very low first gear in an RT6610 with 3.73 rear axle gears.

Also be careful to not overload the front axle. A DT466 weighs about 1420 pounds, which is 500 pounds heavier than the heaviest engine Ford put in an F-250 in 1999, a 7.3 PSD. A 4x4 -99 F-250 with the 7.3 when empty carries 4000-4100 pounds on the front axle and that would be 4600-4700 pounds with a DT466, maximum axle rating is 5200 pounds. The RT6610 also adds an extra 100 or so pounds to the front axle in added weight and that's without an extra transfer case or a driver or any load or anything.