88 GMC S15 Jimmy Vibration

I have an 88 GMC S15 Jimmy with a 2.8L, it has around 165,000. maybe a month ago or so, a vibration started when reaching speeds of around 60, it would stop when i let off the acceleration or if I gave it more gas, or when traveling up any sort of hill. it felt like u joints needed to be replaced. I replaced them and it was still vibrating. took it to a shop and he of course couldnt replicate the vibration because it needed to be going around 60 mph, so he only had ideas, he said maybe torque converter. as time went on, the speed range in which this occurs has been dropping from 60mph down to around 45. I tried driving in non-overdrive to see if that would help and narrow down the possibilities and while in overdrive it didnt vibrate at all in any speed range. the speed range in which it vibrated continued to drop and is now currently as low as 25mph, there are some small speed ranges in between that dont vibrate. it still helps to let off the gas, not as much to give it more, unless I give it alot more all of a sudden. still doesnt vibrate when going up hills. non-overdrive has some vibration but not as bad as with. I think that is all of the info I have, PLEASE help.

Thank you in advance