1999 GMC Yukon V8 Four Wheel Drive Automatic 175000 miles
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My 1999 Yukon "service 4wd" light keeps coming on. Usually the first time i start it for the day it doesn't, but anytime after that (the same day), it will. I've been told that i can't go over 60 mph - therefore i have not been able to drive on the highway.

We've had it to a repair place, and they can't figure it out!

What does it mean, and can we fix it, and is it expensive?

Thanks!
Denese