UPDATE --
I tried to capture the malfunction on video and could not because it's not malfunctioning anymore -- In 2WD with Load Match off, the engine bogs down to a stall under load, as it should, rather than doing what it was doing at the start of the thread, which was for the rear axle to become de-powered while the hydro fluid is diverted with no change in engine RPM.
I'm still heading for deep maintenance and diagnosis in another month or two when I can take the tractor out of service for a couple of weeks. Something just ain't right ... hopefully we can narrow it down.