Todd,
i do find it odd that 2 drive motors would be bad at the same time, one maybe but both?
I'm still in the opinion that the problem may be in the control valve manifold, maybe with the directional solenoid valves or maybe one other check valve associated or a counter-balance valve.
remember, if the circuit is not activated and all valves are closed you get no path for the fluid to go so the motors will not want to turn, your moving the motor against itself which is why they do not want to move. With the unit 'not running' all motors should be this way and have a lot of resistance to them. Not to mention the brake valve being engaged should have all of them locked down unless you have the manual brake release pumped up and in brake release mode.
the fact you have one side of the circuit still allowing the wheels to turn freely when the unit is not running tells me there is some issue causing the circuit to remain open.
Let me do some checking and see if i can come up with some other tests for you to help narrow down what is causing this.
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I went over the hydraulic schematic and the circuit associated with the right front and left rear is associated with drive valve B. Now there are 2 check valves in the drive circuit, valves L and N on the component call-out legend in the service manual. I would pull those out and check them as well.
You will need that schematic and function valve diagram and the call out legend so you can locate the check valves.
Note: you may already have become aware of this but i will remind you that this hydraulic system has a tendency to gravity feed oil from the reservoir when lines and valves are removed and can make a huge mess of oil and literally drain the majority of oil from the tank if left in this state. There should be a one or two(in some cases) gate valves located near the reservoir you can shut off that will help stop this. They should be inline with the suction hose(s) feeding the main pump. It will still have a tendency to still drain out (from the return side lines etc) but not near as much. Just make sure you have a container to catch the oil.