Sounds like the diff is slipping under high loads. Can't think of anything on that truck that might make a sound like that other than a ball diff that's slipping.

Are the spring coils kissing? I think in a previous post you said they were but didnt go back to look, sorry.
Maybe a new spring, especially if you have the older spring and not the vts spring. Even if you have the more HD vts spring, they fatigue over time.
Do you have at least 6 pins? Sorry, again didn't scroll back a page or two.