8-3/4 Posi Whining......

I picked up a Demon with a new 8-3/4 Posi rearend (3:55 gears).......the rearend looks brand new, used very little as advertised.....quiet when cruising.........BUT,

When you back-load the gears by letting off the gas, they whine......quite loudly......I don't know if the gears were maybe not shimmed properly, or just need some better lube or an additive............

This is quite annoying and noticeable........like an old truck........

Pointers?