fwd goes inactive for no explainable reason -- how to fix? on 2007 Mercury Mariner Hybrid

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fwd has always worked normally. suddenly fwd when there was about an inch of snow signaled that it was deactivating. Car is almost useless without fwd. Replaced all tires at suggestion of dealer. Shortly after tires next storm caused exactly same deactivation. Dealer says fwd is deactivating because it is overheating and therefore working properly, and says computer reports no errors. BUT DEALER HAS NO EXPLANATION FOR WHY IT IS OVERHEATING NOR HOW TO FIX IT, (SINCE IT IS NOT BROKEN EVIDENTLY.) Could anyone who knows anything about such a situation let us know. Thanks.
(1) Answer
I'm kind of confused as to what is happening with your Mariner Hybrid.

You are saying that the 4WD system light came on (4WD), and does it have a message about being disabled?

The odd thing is, when that happens, your 4x4 is locked in, so you should have outstanding traction. However, you may be having a problem with the system that is causing your frustrations. I think you are going to need additional diagnosis.