Hey folks,
Just found this thread as I was having the same exact issues. I recently bought a used 425 at auction and didn't notice any issues on my first test cut. Then after setting the traction differential lock all **** broke loose. Yes I need bearings and found a loose tierod connection, and possibly the rod going from the front to back could use some new ends. With all that bad what was making my unit jump was the diff lock being stuck!!! Just think about all the erky jerkyness and what would force it to such an extreme. Notice how the outside wheel is turning way sharper than the inside? This means that the wheels if turning at the same rate with the diff lock on, will become unstable on the ground and fight each other, causing your grass to be chomped up and backlash even if everything is tight.
Anyway, check into it! and hope it helps!
tuned

Just found this thread as I was having the same exact issues. I recently bought a used 425 at auction and didn't notice any issues on my first test cut. Then after setting the traction differential lock all **** broke loose. Yes I need bearings and found a loose tierod connection, and possibly the rod going from the front to back could use some new ends. With all that bad what was making my unit jump was the diff lock being stuck!!! Just think about all the erky jerkyness and what would force it to such an extreme. Notice how the outside wheel is turning way sharper than the inside? This means that the wheels if turning at the same rate with the diff lock on, will become unstable on the ground and fight each other, causing your grass to be chomped up and backlash even if everything is tight.
Anyway, check into it! and hope it helps!
tuned