This won't do anything for gear howl, the friction modifier is only for the clutches in the differential.
OP, if you wait and drive it until next week and it continues to howl in the mean time then it will be too late to fix it as the pattern in the gears will be set. It may already be too late.
And did the shop install the correct inner pinion bearing? It is different on 2010+ cars than on older models and the install kit comes with the early bearing.
I have a little whine on my 3.73 FRPP. According to AM who I called twice and asked, the bearing that comes in the "installation kit + bearing kit" is the correct bearing. If I ever found out that was the cause of my whine, AM would never see me again.
Current Mods: 19x8.5 Charcoal AMR Rims, Bama Tune 93H, Airaid CAI, FRPP Sport Axle Backs, FRPP 3.73 gears, Boss/CS/GT500 Rear Diffuser, GT500 Rear Spoiler, GT500 Mud Guards, Raxiom Reverse/License LED Lights, OEM Take off Black Charcoal Leather, GT500 Steering Wheel, Auto Headlights, Chrome 3.7 Badges, Quarter Window Louvers, Dark Matter Tint
Future Mods: Matte black GT500 Stripe, Black Billet Grilles, Stillen Front Chin Spoiler, Roush Side Splitters, Painted Mirror Caps