My 99 Nissan Frontier did that and it was the EGR valve. Getting up to speed was fine. Running 55 and trying to hold it the truck would shake. I first thought I had a tire out of balance. Hard to hear those 4 cylinder engine. Accelerate and engine runs fine.
The Nissan has a tube with threaded plugs in it. I took one out, filled it with sea foam while the engine was hot. Let it sit over night. Didn't see much change but in a few weeks it was back running fine. The sea foam made lots of carbon come out the exhaust for about a mile too.
You may have another problem but it sounds like EGR.