Gear/grinding Noise

Need help narrowing down some gear noise. So far if I disconnect the rear driveshaft and run the Jeep at speed on jack-stands I get no noise. With the rear driveshaft connected and any other combinations I try I get the noise starting around 2000 rpm in 4th. U-joints are new but they were cheapies. So, could it be U-joints or probably the transfer case that just quiets down with no load on the rear yoke? There is no noise at all if I run it with the TC in neutral so I think that eliminates the trans. This is a T15/D20 factory combo.

 

rear axle bearings.