Just guessing here, and in agreement with Chief.
Get the thing near an outlet, and run a Salamander(Torpedo heater) on the lines and see what happens.
Old school Hydo fluid is somewhat hydroscopic, and over time can collect moisture and then freeze.
Go with the obvious first, and heat that puppy up.
After that, it's all about any interaction the two systems might share, and tracking downstream.
Lemme guess, brake pressure releases the diff lock momentarily?
On the bright side, it ain't an electrical problem