1999 Chevrolet Blazer

Every time the blazer is in drive there is a loud knocking noise coming from the back end. The noise only decreases when turning or when you let off the gas. I have had the universal joints replaced and it didn't change anything at all. The blazer is still am king the loud knocking noise. Do you have any idea what could be causing the knocking.
September 5, 2013.

Has the differential itself been checked? Is the gear lube full? It could be a spider gear that is damaged.