is this my t-case?

I've been trying to track down this annoying noise/vibe since forever. At first I thought it was my pinion angle but it wasn't. Then I thought it was my cv shafts but it appears not to be. It's an annoying grinding-like resonating noise that I feel through the cab on acceleration or deceleration. It starts on a moderate acceleration around 20-40 mph. If I gas it hard it goes away. It ALWAYS grinds when going up a long hill. I was coming down this hill today and put it in 1-2 gear (xj 231 with D-3-2/1) to slow down and when it got slow enough to shift into 1st on it's own, the grinding occured really loudly. Once it got under 20 it went away. I can feel it lightly when coasting down a hill at around 45 mph.

I have no front shaft hooked up and it's doing this. I swapped rear shaft to front and it got SO MUCH louder right underneath me when i drove in 4wd (front wheel drive and no rear shaft). I could definitely tell it moved to the front when i put the shaft in front.

I used my hearing aids today to try to hear anything when going up the hill at the beaver valley mall and during certain mph it would make this loud clicking or rattling type noise. I'm surprised I heard that too. Made me think of the cat converter but I thought those would make noise at idle.

I don't know what to check. I have no seals leaking and both pinions are nice and tight. I have the PORC sye installed and been on there for about two years now.

any ideas?