SC2400 jumps to high range

[temmert]

Hi everyone, I bought a used Cubcadet/Yanmar sc2400. I think it's a 2012. Has 425 hours. When under a load the range selector shakes hard and transmission jumps to high range. Done a ton of research and found a few post about it, but no real solutions. I check the fluid and it is full. Anyone else having this issue or can offer me some guidance?
THANK YOU