Hi, I think I would look at the speed sensors first. Maybe they just need a cleanout. They can't be too bad if the 4-Matic engaged light only comes on at those speeds. I had to change both front sensors as the light came on almost constantly and gave me problems with the ABS as well. Now all is ok except for erratic steering sensor input when turning.
I don't think it will do any harm to the 4-Matic system as at that speed you should only be in "Stage 1" ie. 35% front / 65% rear and would in fact probably have enhanced road holding as well. You didn't mention if you could feel any unusual forces through the steering wheel when this happens.
Another thing that could cause this would be the rolling diameter of the tires. If you have different tires back/front it is possible that they behave differently at high speeds. Just a thought....
Tom