Depends upon what type of ranger and what your goal is.
Personally, if your axles are geared right for the tires, then the ranger underdrive seems a waste. IMO, better off getting a true doubler such as NP203, minitruck, etc.
FWIW, I LOVE my NV4500. While the 1st gear granny low isn't useable on road, the 2nd gear works perfectly for take off. With 35" tires and 4.88 gears, it does pretty good on the freeway. Probably geared a hair low. But next set of tires may be 37"ish so I'm set for gearing