The following is a quote from Bill Woboken in response to a similar question a while back on another board. Bill is a friend of Jerry and did work for Ford as a driveline engineer.
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Today's Lesson: Self-Control.
BillW
There IS a break-in period for rear axles, or a new gear set.
Drive at varying moderate speeds (<60 mph) at moderate loads (NO WOT!!)for about 500 miles.
Change the fluid. If it looks like black metallic paint, you will understand why I suggest this.
If you do not follow this procedure, (aside from a crappy pattern emerging from brinelling/sfc checking on the tooth profile, and run high highway speeds and do hard shifts (big, short duration torque reversals) with either manual or automatics set up per Jerry's thesis,) a phenomena called "bump scoring" can occur. This is actually a breakthrough of the hardened surface to the soft, chewy center of the tooth, the part that provides strength but not wear resistance.
I have actually tested under extreme conditions (120F ambients, 80 mph cruise for 40 miles on a "green" axle) a Mustang Cobra 3.27 axle which actually got the axle lube to the smoke point (>400F)and very close to flashover. Now, subtract the appropriate amount from the 120F ambient to a typical summer day and you'll still find that the axle lube overtemps.
This overtemp condition results in a breakdown of the lube layer and can do great damage to the tooth finish and even close up allowances for expansion vs normal tooth mesh....GRRRRIIINNNNNNNNNDDDD! !!
Hope this helps.
BillW