Baystate Jeepers

Has anyone ever had an issue where the drivetrain has a vibration when not on the gas, or not on the brake? Both of those things put the drivetrain under load. When coasting it's not under load, and that's when the vibration starts. I got the jeep back from Offroad Elements and it was perfect for the first ~150 miles. Today on the way to a business meeting from Clinton to Burlington MA, about halfway on the way there I was cruising at ~70 on cruise control. I hit the brakes and started coasting as traffic tightened up and I started to feel it, As I progressed I noticed that it would stop on acceleration or heading up a hill, but as soon as I crested and let off the gas it would start vibrating. Made it there, but on the way back home it got a little worse, now as soon as I got off the gas it started vibrating both the steering wheel, and felt it in my seat.

My first thought was Drive shaft, was either unbalanced or not tightened/something loose, or maybe the angles were wrong. but they are both brand-new Adams 1350 shafts just installed. My next thought was maybe a control arm bushing, I'll be checking them all this weekend, when I'm doing the Diff Covers and changing all the fluids (Engine, Transfer case, Diffs, and maybe the Brakes (never changed in 12 years), and greasing all the Zerk fittings.

Does anyone have any experience with something like this? Any leads?

Even when you aren’t on the brakes but off the gas you’ve got engine braking. You’ll need it in neutral to verify vibrations with no load or if they are drivetrain related.

Confirm your new shafts have enough length but aren’t too long, you’ll need to pull the shaft on one end at least to confirm where in its travel it is.

Check all the ujoints on both driveshafts, but also check your axle shaft joints, those are always spinning.

Do a simple lug nut check, eliminate the “stupid” stuff.

Turns out it was "the Stupid Stuff"

I found the issue, The Lugs on the rear Driver spacer were "loose" and one lug snapped off.

Everything was tight and working great for the first ~100 miles from the shop. That's when the vibration started (on a 40-mile round trip for work). But now the Spacer bolts are toast and won't hold the lugs on at all anymore.
I torqued the remaining 4 down and had to run to the hardware store about 1/8 of a mile from my house, to pick up some split washers for the Diff Cover, Figuring that 4 lugs and driving less than 30 MPH for a 1/4 mile round trip would be safe. I got the the hardware store and three of the four were gone and the 4th was holding on by 1/4 a turn.
Offroad Elements is sending a flatbed and they are making it right. They are going to assess the damage to the spacer and either replace them all or change the bolts on the spacer as it's got to be the threads that are allowing them to come off. Both times at the start the lugs were fully torqued in and seated correctly, but 2 times the lugs worked themselves off almost completely. Also, they need to look at the rim and make sure that the lug holes in the aluminum have not been damaged.