3rd gen - annoying steering wheel shake at 65mph and vibes down low

This is going to be a running log of my troubleshooting to fix a problem and solicit input.

I recently had 4" spindles installed on my 2018 with 20k miles (2WD) with an alignment from a shop I trust. It honestly rode PERFECTLY right away. Shortly after, I added 1.5" add-a-leafs on top of the rear blocks for 2.5" rear lift to level out with the front. Around that time, I added quality hub-centric wheel spacers as well. You can say I lost track of my 'baseline' because I did a few things within a few days so unfortunately, I couldn't cite one 'thing' that was the cause.

The next thing I know, I have this terrible 60-65MPH steering wheel vibration/shimmy, and the vibes through the floorboard from 10-30MPH. I immediately focused on the spacers and tires. I had the 285/75R16 tires road-force rebalanced TWICE. The vibration persisted. Moved tires around, removed spacers, the vibration persisted. Cooper Tires, in their generosity, offered to replace my tires since they were within the first 2/32 of wear and we all suspected. BRAND new tires where mounted and road force balanced and the vibration persisted!

I KNEW that the vibes through the floorboard were the driveshaft angle. I was still uncertain about what was causing the 65MPH vibes.

Using great guidance from this site, I measured my driveshaft angles, here:



I ordered Toytec 2.5 deg. shims which will get the pinion angle to ~ 2.3 degrees which is around stock. Then I'll play with the carrier bearing if needed.

I COULD NOT BELIEVE, no how, no way, that the driveshaft angles were causing steering wheel shimmy/vibration. SO, imagine my surprise, when I'm going 65MPH on the freeway and the steering wheel is shaking badly, and I drop to 4th gear and the steering wheel vibes GO AWAY. Basically, by dropping a few gears, I changed the driveshaft speed which changed the load/frequency and the steering wheel smoothed out.

I still can't piece it together in my head. As far as I can tell, the driveshaft vibes must be traveling up to the motor mounts or chassis and causing some inbalance of energy transfer side to side which is being picked up in the steering.

I will update this after the shims are installed. In the meantime, people may say you're crazy for thinking a rear driveshaft can cause steering wheel vibes and they'll tell you its the tires like I was told 50 times. If you're lifted, especially over 2", do the math and get your driveshaft angles right.

 

seems like this is the answer

 

What do you mean?

 

Once you added the 1.5" add-a-leafs on top of the rear blocks for 2.5" rear lift to level out with the front and everything after you got the wheel shake.