Chasing vibrations (lots of reading)

I have an 1990 cherokee with the 4.0, aw4, np231, dana 30 and c8.25 with 3.55 gears and about a 3 inch lift. I get this vibration between 35 to 55 when I have it under load. As it soon as it starts vibrating if I take my foot off the gas or slip it into neutral it goes away. So far I've done the following: replaced all u joints, hub assemblys, rotated and balanced tires, replaced motor and trans mounts, had rear driveshaft balanced and tested, taken out front driveshaft and still vibrates! I've taken my front drive shaft out and did the reverse and taken my rear driveshaft out and drove with just the front ds in 4 hi, and didn't get rid of it. I should include that this started happenin after I got my 2nd transmission installed but it shifts fine. Since then I've had to replace my transfercase and I did a rear end swap but that didn't fix anything either it still vibrates. I know its not my lift because I've had it for 2 years with out any problems. Also I've aligned it and it has fresh new steering parts, so its not death wobble. So what have I missed or what should I check next? I'm running out of ideas and I would like to fix this while I have money. thanks for any help guys.

Have you actually checked your driveline angles? T-case drop, sye.. degree shims.

I have a long time I go I can't remember the exact specs but I believe it was at most .5 degree difference from the t case to the pinion flange. And like I said I don't believe I need shims, t case drop, or an sye because I was driving around with this lift for a good year in a half with no vibratings or any problems.

Yea, but there's a reason why people do a SYE other than to just get rid of vibes. The angles put unnecessary stress on the output of the t-case. It ends up wearing the output bearing and can further damage internals of the case afterwards. You could be feeling the deficits of the effects of having poor driveline angles for an extended period of time.

That's why people opt for a SYE, to avoid damages like you're experiencing.

Thats true, I guess I just thought I wouldn't need one since I only had an 3 inch lift. I could invest one. But could this be transmission related also? I took out the fuse to the trans computer and did the manual shift and it seemed not to vibrate. So could that mean it could be something electrical with the trans. I would say it seems to vibrate when its in 3rd going to 4th maybe? I'm just wondering cause like I said, these vibrations didn't show up till after I had my transmission replaced. Thanks for the help so far guys.