1998 GMC K3500 7.4L vibration

[Rob Petrie E395]

I've owned this truck since almost new (less than 3000 miles) From day 1 it has had a very slight vibration around 2200 Rpms. It was a executives vehicle. He got upset about the vibration. And dumped it at the dealer I worked at.. The vibration never bothered me. I just figured it was something internal and eventually it would get worse and I'd either rebuild it and balance it or put another engine in it. Fast forward 20+ years and 150,000+ miles. All the sudden one day my buddy borrows to haul calves. And it gets bad enough to be concerning. While checking it out I noticed that it seemed like the engine moved a lot. And the drivers side motor mount may be starting to bad. So I get one and I'm hurt so my buddy comes over to my shop and puts it in. To my surprise it 100% cured the vibration. I was like I've been driving this thing for 20+ years like this and it was just a motor mount. Or so I thought. 3 months later out of the blue its back with a vengeance. But its different its vibrating on deceleration where before it was on acceleration. Even sitting still in park it vibrated with the rpms going up not down. We look at it and it seems the right motor mount may now be doing the same as the left. If you've ever changed motor mounts on one of these things you know why both were not changed at one time. Its not a fun job. So my buddy comes over and changes the right motor mount. Its cured again. Or so we thought. 3 days and a couple hundred miles later its vibrating again. So either were getting bad parts that look perfectly fine. Or something else is wrong. If anybody has any ideas before we fight another set of motor mounts into this thing I'm all ears.