Are you sure it's coming from the diff? Your detroit sounds like it's fine. That's how they are supposed to work.
A short wheel base truck or 4-runner with dual cases will almost always have driveshaft noise if you don't run a CV on the rear shaft. Makes a noise just like you described at part throttle and decceleration. Nothing to worry about if you can live with the noise.
I hear this same question all the time. People do a dual case setup and now they have this noise they didn't have before the dual cases. Most people usually think it's the t-case. Very common problem with a short rear driveline.
On the other hand you could have burned up a bearing in the t-case, but I'm betting it's just driveline chatter due to the shorter rear driveline.
