HELP!!!!! Rear Axle making a low humming noise

[WNY964x4]

i just installed a new rear axle in my truck and now at speeds above 40mph it makes a very audible humming noise , when i let off the gas it goes away , step on it again it comes back

here are the stats on the truck

Year: 2000
Front Axle: Dana 35 , 3.73 gears
Rear Axle: 8.8" , 3.08? Gears , Trac Lock
Transmission: Auto
Wheelbase: 126"

regarding the difference in gears , i was told by the member who sold me the axle that it had 3.73 gears , and come to find out it does not , i am not exactly pleased.

also , i installed belltech shackles and springs for a off road package ranger , so the back end sits higher , and the u-joint looks to be right around 0 degrees

 

[Sasquatch_Ryda]

Could be worn gears, a bad contact pattern or bad bearings in the diff.

 

[WNY964x4]

the rear has all new bearings , seals , clutch pacs , springs , cross pin and lock bolt

but that is according to the member that sold it to me , who also told me it had 3.73 gears

this is really stressing me out