not sure how they rear diff is explained in that situation being that the ring gear throws the rear end fluid up and into the reservoir of the front pinion bearings. The outer wheel bearings would still be fed from the round tube that is hit with fluid also being flushed into them from the reindeer doing its thing. There's really no good reason that the rear diff would be bad unless the front end were jacked up so high that you filled the cover with grease and even then the ring gear would still be throwing it.
The reindeer should handle 75 mph with no problem. Santa travels much faster.
Bruce