Good ole grinding 55mph and up from what seems to be the transmission...

Glad you fixed it. Your not going to want to hear this but the ride height should of been the first things you checked when this problem came up. Not blaming you but the shop should of checked this right a way if they are worth any salt. No just throw parts at it. Crazy.

 

Yes I was going to comment sounds like the mechanics are not that experienced as they should of been able to go through what we call the process of elimination, e,g the computer don't work, first thing we check is the power turned on etc etc.

 

Ride height can be very tricky and time consuming. This case valve itself was bad. I’ve had to reset mine on my Classic, more than a few times, after adjustment. It likes to move a bit when tightening the nut.Best way I’ve found is to check it with Trailer and without. Pulling Truck from under, and back not quite hooking it, with a heavy load. Using dump valve. ETC. A bad valve will give 2 different measurements. Last time I set mine, thought valve was bad. Finally got it right.Still doesn’t hurt to check it down the road. The setting on mine gives a 1/2” fudge factor. I’ve tried the max 2.75”, due to the long wb. My theory was wrong. Setting it at 2.50”. the minimum is too low. 2.5/8” is right on the money. Using a small piece of cardboard cut to 2.5/8”as a gauge. Measured at right front axle, bottom of frame to top of spring bumper block. I also make sue the other side of the axle is within the 2.50-2.75” range. It’s temperamental. Has to be just right to ride smooth. WS w/ Hendrickson was the same way. Had to be just right.

 

Didn't dawn on you but should of with the shop.

 

I have that same problem I’ve replaced my leveling valve but I have not dialed it in really I have a video of what the sound is when letting off the pedal

 

Too high, it grinds, or sometimes just chirps occasionally through the shifter like a tuning fork. when accelerating

 

Leveling valve set way to low will do this

 

Or .001” too low if you have AG380. That suspension doesn’t have to be out very much before the shaft angle shakes blur your vision.