not an expert like the Gurus you have on this thread but based on experience of using Highway, All and Mud Terrains would suggest the following before going for any mechanical repairs etc.
The issue is a sum (or either) of;
1. used Mud Terrains, which are hard, might have been used i.e. abused properly in life of Mud/Rocks. A 2018 MT that costs alot would not reach lhr markets by 2019;
2. maybe slightly bent/worn rims and it is common on old rims; and
3. weak suspension Parts (bushes, ends etc) that are weak but would not be an issue with softer highway terrain tires.
If you have a friend that can lend you his HT tires n rims of a similar vehicle, swap and drive for an hour to ascertain the cause.
Another way to confirm the above is, how was the ride before change of tires? were the rims checked by wheel balancing shop? and what are the weights being used to balance as used MTs are difficult to balance? the answers to these would help you identify and solve the issue. Its easy to double check the rims and tires at another wheel shop.
Vibration from rear can be due to the weaker tire, rim or bushes etc. are the rear end or just a feeling that it starts there and then takes over the whole vehicle so difficult to identify exact spot.
Vibration at slow speeds may be due to the impact time the weak part gets to surface and as the speed increases it gets lesser impact time and leads to lesser vibration.
If tires are the cause it would be good to change them before the possibly weak suspension parts start to fail.
The easiest solution would be to Trade In the MT tires and get Highway Terrain tires. While doing that do not solely rely on the manufacturing date stamp (coz can be tampered) on the tire and check for softness and dryness before you commit a purchase. Would be to great if you can New Highway Terrain tires.
