Drive line or rear diff noise upon letting off throttle or getting back into

What is your question?

I am driving at 30-40 and let off accelerator then get back into it it feels like 4WD is kicking in. Had it looked at dealer they said u-joints and driveline carrier bearing look good. Also said rear diff mounts looked ok. I do notice that the rear diff looks like at one time or another was leaking fluid at the output seals but has sense dried up with soil deposits on both sides. Just getting frustrated with it. Any advice or suggestions would be appreciated.