rat4spd

The issue he is alluding to is that the orientation of the front axle is backwards compared to the rears, therefore, the rotation of the ring gear will be in reverse compared to the rears to make the front tires go forward. Ring gears are made to primarily turn one direction because of the cut of teeth.

If you simply swapped a rear diff in, the ring gear will have the backside of the teeth loaded instead of the front. I don't know what the Rockwells do for the front, but that is the concern.