06 379 grinding issues.

I have a 06 379 18 speed with the 3rd axle as a drop axle. The issue is this, driving empty its not that bad but loaded and your in the pedal you don't here it but as soon as you let off it sounds like it is grinding from the transmission, kinda like the leveler is out or maybe carrier bearings. Now the leveler looks fine the carrier bearing look new. the bags look good. The company said they have had the truck and different shops and cant find anything wrong but I don't know if they really have took it anywhere or not, There is something going on somewhere. Any ideas would be great,

 

Well the truck was bought used and came back with a clean history but its suppose to have a 20k axle up front from the factory but it has a 12k instead and I dont know the truth behind this but we have been told that it doesnt have the motor that came it in it from the factory either, I guess ser# dont match. I will check the mounts and see.

 

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