Taken from the: "2004 DR Ram Truck 1500-2500-3500 Technical Publications"
Schedule "A" and schedule "B" the category which I fall into.
Change rear axle fluid. 15000
Change front axle fluid (4X4). 15000
It then repeats it at 30000 45000 and 60000 miles etc etc.
So the front does get changed at every 15K with the rear.
If you think about it, even though people do not use 4x4 all the time, when we disengage our transfer boxes to put the vehicle back into 2wd mode, the front diffs and axle on a 1500, are still turning. So, the axle is doing the same amount of work as the rear differential, all be it, not under load.
A lot of people have this misconception, that fluids need only be changed every 15K for the front diffs, as they don't think the diff's are doing much work.
I personally change it, just for piece of mind. I already have had the stock LSD junk on me in the rear
and that was REALLY expensive over here in the UK. So I'm not about to let the front go boom, for a few quids worth of fresh oil.