I’m happy to read that you have discovered what may have been the cause of the mystery vibrations.
For me, over the years tires have been the major and most common cause of vibrations. The obvious is out of balance and the less common is out of roundness. Flat spotting can certainly be an issue for vehicles that are parked for a month or longer.
Some years back I had a tire shop put a new set of tires on my car. I noticed vibration immediately and drove back to the shop. I asked the shop to verify that all the tires were balance properly. They checked and rebalanced the tires. I drove the car from the shop, went out the freeway and the vibration was still there! I took the car back and I said something is wrong, the vibration is still there. The shop at first wanted to blame the vibration on something other than the tires and their perfect balance job.
Flashback: My dad was an old school auto mechanic, and back in the early 60’s of buying “retreaded” tires there was something he told me once and I never forgot it. He had a new set of retreads on one of our vehicles and apparently they produced a vibration after balancing. I remember him telling me that “You can perfectly balance a tire the shape of a football on a tire balancer, but it will bounce like a football going down the road.”
Back to the shop… I asked the shop to verify any possible out of roundness. Sure enough… two tires were not in spec. One tire was visually noticeable as it slowly spun on the balancer, I watched it. I asked that all 4 tires be removed, and I had them replaced with a different brand.
On both my wheeling trucks – first 1986 Longbed and my 1985 shortbed, I always ran with lower tire pressure than spec’d. On my 1985 shortbed I ran 26 to 28 in the front and 22 to 24 in the rears for on-road as my daily driver. My 85 only weighed 3200 lbs. I rotated and rebalanced the 32s and 33s, about every 5K to 8K miles depending upon the tire wear. I never experienced any serious tire cupping.
Gnarls.
