Rear cab mount bushings.

Look around under the cab, can’t be hard to find. Chances are there’s only the 2 air bags, and a torque rod. Check torque rod bushings, air bags for rot, replace shocks if needed.

 

Sounds like an air bag issue. Assuming it has air ride cab. The torque rod keeps the cab from moving side to side. Each axle has one also. The one on the cab is smaller than the ones on each rear axle. A bad bushing in one could cause a lean. I’m not familiar with International. Usually only 1 torque bar for air ride cabs/sleepers. Volvos have 2. Look under the cab/sleeper. Should see a bar running from the bottom of the cab/sleeper to the frame. I’ve seen them snap at the frame. Not much else will cause a lean. When I bought my Truck, it had a bad torque rod bushings and one wrong cab air bag. It leaned and wiggled a lot. Lol. Easy to change inexpensive items.

 

Check the air ride leveling valve also. It may need adjusting. Air ride may be low, basically non existent, making it lean.