Something has to give. The average wheel circumference between the front and back of the car will never be "exactly" the same. If the clutch pack is fully engaged there will be a difference in wheel speed between the front and back of the car. Either the wheels spin just ever so slightly, or the clutch pack slips. Either way there is a HUGE loading on the front and read diffs - let alone the drive shafts, CV and uni joints, tail shaft, transfer case drive chain etc. Normally a diff will just left go. The weaker of the two is the front diff so in my experience that would be the one that blows. I'm sure the rear is showing signs of wear. That is why you never ever run a run a 4x4 (with no centre diff) on tarmac, not even for a second.
Edit - or maybe he got lucky and everything is ok, just the front diff blew. As an engineer I would have to pull it down and rebuilt it, just to put my mind at ease lol.
Jeff