ok after looking at the hydraulic diagram the 'B' variable control valve on the manifold control these motors.
I would start there and check and make sure that control solenoid valve is getting signal at its coils.
You can take the control wires off the working solenoid variable valve for the left front and right rear motors and move them to this one and see if the symptom changes, this will verify if the valve is working or not.
If the symptom does not change then you may be looking for a stuck check valve in that circuit or maybe a valve with a broken oring.
I know using the diagrams in the manual can be confusing because they may list 'letters' for each component in the diagram but the corresponding information is always listed in another section explaining what the letters are. You just have to spend some time looking through the manual to find this information.

Just remember the orientation of the controller will remain the same so you will have to keep this in mind when working the control handle. And always make sure you have the wheels off the ground when testing so you will not have any unexpected movement of the machine. Or you can disengage the wheels using the lockouts on each wheel hub to prevent machine movement, but locking out wheel hubs does make it harder to tell if the motors are working correctly.