How much does the drifting actually affect your operation of the tractor?
There are times when people are not realistic, and expect nothing to move,EVER. To me, if the rod extending affects the operation of the task being performed, then it needs to be addressed. If your main issue is when bush hogging, then simply disconnect the rod end coupler of the side link cylinder. If the rod extends when grading, then you need a remedy of some sort.
VERY good chance that the control valve is faulty.
But just to be sure, with an implement attached and the implement on the ground, disconnect the couplers. Mark the rods next to the cylinder seal with a felt pen or a piece of tape. Raise the implement. Check in 1 hour how far the rods have extended on their own. If it is less than 1/4", (probably less than 1/8" in reality) the cylinders are fine. Your control valve-s are then your problem.
If you have a GREAT dealer, they might check it out. Kubota Corporate will not address this problem. " We're sorry, but that's how it is" is their answer to this defect.
They will not fix the problem.
There are 3 solutions that keep the hydraulic cylinders in place.
First, cheapest and an all around PITA is to simply disconnect the rod end coupler when not in use. This gets old real quick.
Second, install DPOCV (check valves) on the cylinder-s.
Problem with the check valve solution is if you happen to have and use the float feature, that feature is no longer an option with the controls.
3rd option, install an electric on-off valve at the coupler for the rod end connection. Valve is closed, the cylinder is rock solid. Need to adjust or use the cylinder float feature, then the valve is left open.
These are the 3 ways that I know about to deal with the issue and still have hydraulic adjustments.
I recommend the electric valve option if you have the float feature with your control valves. No float feature, go with the check valve-s.
Good luck.