UPDATE******I have a 2021 unlimited rubicon with the same issue. I have put more miles on my jeep bringing it back and forth from the dealer than just driving it. It has been in the shop for a total of 5 visits for 12 days. The tech's take it for a test drive and feel the vibration but have not been able to figure it out. When I pick up the vehicle, they say " we couldn't find anything, it's normal ". If it were normal, why don't they take it for a test drive and come right back and say "it's normal ". Instead they all recognize that there's a problem and keep the car for days trying to figure it out. I requested to go for a test drive in another Rubicon and when we got it out on the freeway, it DID NOT have the same vibration so how can my issue be normal. I'm really surprised that Jeep isn't taking this issue more seriously and coming up with a fix.
I believe there was some talk earlier in the thread about the wheels causing the issue as well.UPDATE******
The dealership has finally fixed the issue. They had to replace all 4 tires and the rear rims and the vibration is gone. I think it's the rims that had some type of flaw. What's really disappointing is that Jeep would not approve and pay for the fix. The dealership stepped up and paid. They changed the rear rims just to see if it helped and it did. They provided brand new tires because they didn't know if the vibration had worn the tires in a funny way and didn't want to take a chance. I know a lot of people will say that this is not the issue with their jeep, I'm just saying that this is what worked on mine. Good luck!!
That would be nice, but I've noticed the thrumming when coasting in neutral, at idle.I wonder if it could be engine mount related. Someone posted in another thread that there is a redesigned part for 2021.