That question was already answered in this very thread. Page Up....

 

You can just do the rear, but if you turn on 4x4 the front wheels will be skipping along the dirt and the truck will be fighting itself. The front wheels will turn at a slower rate than the rear wheels. I'll stop just short of saying that there isn't one shop out there that would do just the rear on a 4x4, because you never know. Just don't try it.

 

I have 35's and will be going to 37's after gear change. Truck was downshifting alot going up small inclines on highway.

 

Actually it's common around here. But it's usually lower in the front. For sand driving a lot of guys use different ratios fore and aft.

If you don't have a locking differential it works fine.
I've had one jeep that way (pickup, off road mostly) 5.27 fronts with 4.56 rears, and a Scout, 4.56 front, 4.11 rear.
Sand driving the combo works pretty well.
But then, I have owned at least 15 4x4 vehicles (I'm old) and have yet to need 4 wheel drive on the pavement, where you want the ratios to be the same to prevent binding.
Chris