I've actually done this...
Well, I do it for winter. With all the mud and snow out on the ranch, the huge contact patch on the rear end doesn't do much for traction. With only 2 tires on the back, it'll sink down and grab some more solid mud under the really greasy stuff.
Of course, since the front tires track right between the rears, either taking off inners or outers leaves the rears riding on the edges instead of in the ruts of the fronts. So, get a set of stock 2500 wheels ($30 used at a local tire store, call around). I use the dually offsets on the front and the 2500 rims on the rears with a set of pretty aggressive mud tires.
Once we get back to "normal" dirt/mud/gravel, I switch back to all 6 tires again...
-cj