Transmission grinding / chattering when going downhill

I drive a 2011 peterbilt 384. When going from 40 mph to 50 mph it shakes, A LOT. After 50, it smooths out. Today is 2/27. I got new drive tires on 2/25 on the front drive axle. The back drive axle tires were put on in august 2019, six months ago. My boss replaced 3 of them, the other one has decent tread but is a little more worn than the other three. My steers are less than 9 months old as well and still have good tread. What ideas do you have that could cause this shake at 40-50 mph? Anyone got any ideas?

Second issue: After I got the new 4 tires on my front drive axle, the transmission makes the grinding noise every time I let off the pedal or coast down a hill. New tires shouldn't change the ride height, but I'm starting to think my ride height is messed up. Any thoughts?