The "rumor" I heard was that you cannot use the Gear Vendors in 4 wheel drive but, the US Gear unit will allow 4 w/d use.
Also, if you want to use an exhaust brake the Gear Vendors doesn't work well with them. Has something to do with the cone-shaped clutch, I believe. Apparently, it works well in a forward direction but, when you try to reverse the direction on the clutch, it doesn't want to hold very well.
I am not an expert on this stuff but, these are things I have heard. I think it would be a good idea to research this to make sure you get the proper unit to fit your needs.
Joe F. (Buffalo)