There are two aux O/D on the market. GearVendor and US Gear Dual Range. I have owned both in a PSD. The GV unit is a small single piston automatic tranny. In Auto mode it will upshift into "overdrive" around 50 mph and down shift out at 25. It will not operate below 25 mph because it is not building enough pressure to shift or hold the clutch pack in place. It does not like Exhaust brakes or downshifting because it won't take the "reverse torque".
The US Gear unit is a small electrically shifted manual tranny. It will operate at any speed and exhaust brakes and down shifting are not an issue.
The GV unit mounts behind the tranny and or T/case and as such will only work in 2wd and not in reverse. The USG mounts between the tranny and T/case and will function in 4 or 2wd and forward or reverse.
Either one will work with both automatic and manual trannys. neither will "split shift"by themselves but you can do that manually by toggling the switch. the GV unit is much more sensitive to abuse and maintanence. it uses a special GM oil that is fairly expensive and they recommend oil changes ever 5000 miles if you tow alot.
USG use reg or syn gear oil and service as required. the USG unit has a higher tow rating and the warranty doesn't have any restrictions about weights or commercial use.
hope this helps. like I said before I've owned them both. The GV unit was OK but when I went to order one for my current truck the GV tech said he really didn't want to sell me one cause my truck had "too much" power. Its a mostly stock F550! the USG unit has been absolutely flawless for the last 30,000 miles towing a 15,000lb 5th wheel around the country.
Barney
