Glad you figured it out!

I had a similar issue with my
1998 1988 Silverado. Shortly after I bought it in 2001. I would hear a rhythmic thump, thump, thump, low in noise level.
I asked during the test drive about the rythmic noise.
The dealer "
Emerald Motors" said it was common the that year of Chevy.
Within a short time, it got much worse.
When I turned to the left it would get even more pronounced.
Had my Spicer U-joints checked out at an independent shop. They were fine.
Then we opened the rear end, and multiple teeth fell out. It had a chunk missing with many teeth from the ring gear. The weird part is the gear oil was pristine, no noticeable shavings.

So the dealer, that I bought it from
"Emerald Motors" on meridian in Puyallup, I can only infer,
Took the rear end cover off.
Saw the problem, and replaced the broken teeth back in the punkin, and refilled the gear oil.
Definitely tried to cover it up.
I confronted him about it, and he lied his ass off, then when called out, threatening to call the police. Said he felt threatened.
Well I suppose he "might" have felt threatened,

I can neither confirm nor deny bodily harm was implied.

I had a detroit locker installed.
Still does not dismiss "Emerald Motors" and that snivelling owner are a ripoff.I stand by that claim.