I did the Ford Racing 3.73 gears on my 2009 Mustang
with the V6 auto as well as the trac lock and I've been totally happy. I tried the Motive and Richmond from American muscle first but they whined too much. Tried 2 of each same result. AM stood by and covered all costs invloved. As soon as the Ford Racing gears went in the whine was gone.
Much quicker acceleration and only went down 2 mpg to 26 highway from 28. Well worth the minor drop.
Depending on other mods I have heard others consider upgrading to the 8.8 rear end. But to be honest I have modded my V6 heavily and I have had no issues. I think mostly because I can spin the tires very easily so there is little worry about breaking anything.
So far I have installed:
Ford Racing dual exhaust with crossover
Ford Racing CAI
American Muscle BAMA tuner with tunes for life
Ford Racing 3.73
Ford Racing Track lock diff
Explorer Express HiPo Supercharger
Shaftmasters Aluminum driveshaft
All working in harmony no issues and lots of smiles!