Depends on the year for the differences. The Grand Wagoneer tended to have nicer interior and woodgrain. In 1986, the Grand Wagoneer got a interior makeover and the dash got a little more modern and some other things changed. Here is a breakdown of the axles:
1974-1979 Wagoneer, front and rear dana 44's (some 1974-1975 had drum brakes) (rear 44 is offset to clear the fuel tank) (front is passenger side drop)
1980-1983 Wagoneer, front dana 44 and rear AMC 20 (it is NOT the weak 2-piece axle type like the CJ's) (front is drivers drop and remains drivers drop until the end of production in 1991)
1984-1985 Grand Wagoneer, front dana 44 and rear AMC 20 (front 44 has a vacuum axle disconnect, try to stay away from this one, swapping it out or removing it and changing the axle shafts is an easy fix though)
1986-1991 Grand Wagoneer, front and rear dana 44 (rear dana 44 is mostly centered - just a little offset)
Hope this helps.