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.
Yea i made sure they knew about that. and you are probably correct about the gear pattern. it already has like 35+ miles on it.