I want to install an aftermarket over drive in my truck. First question is, is it an aftermarket over drive or a gear splitter? I have heard them called different things. Also, which one do you all recommend? The only two I know of are from Gear Venders and US Gear. I am leaning towards the Gear Venders model.

http://www.gearvendors.com/f2wd4s.html

http://www.usgear.com/dual_range.htm

Are there any other options out there I should consider? What should I know before entering into such an expense?