IF you think that was the cause then I would for sure replace that fluid. Heat is what kills that fluid. I haven't looked close at these diffs but if they have a drain plug it won't take long to drain and refill. If it doesn't have a drain I would consider getting a pump and sucking out as much as possible.
Mine is probably about due, I should look and get it taken care of this summer.
Speculation alert!
Reading from my owner's manual page 127. There appears to be two different temperature related warnings. The one that I've seen once on my vehicle reads, "eLSD Reduced Torque". Without retyping the "Action" in its entirety, the gist is that if you have unequal diameter rear tires or have done something else to heat up the rear end, this message appears, and you should fix the unequal diameter situation, or cease the bad stuff you're doing that is heating the rear end because, the computer is dialing down the clutch pressure and maybe reducing power train total available torque by dialing down the boost. The next message (I haven't seen this one) "eLSD Temporarily Disabled", this one probably comes on if you didn't pay attention to the first message and the diff got hotter. The action on this one tells you to pull over and take a powder, to let the diff cool down. AND from a safety standpoint, they are warning you that you now have an open rear end that will not help you in a bad yaw event or poor traction event.
There are more messages, one that tells you when the eLSD is restored to working condition (presumably once temperature returns to normal). There is also a "you broke it" message and a "get it checked" message.
I would expect that the escalating "temporary" deactivation messages are intended to preserve the differential fluid, and if you pay attention to them, your fluid will be OK. Obviously, if it tells you it's broke and/or to get it checked; you've gone too far.
Note this would be a really good opportunity for someone in Ford Vehicle Engineering on the Expedition/Navigator (who should be monitoring these posts) to step in and enlighten us all. Just saying.