Question:service 4wd light comes on

I have a 200 Chevy S10 Blazer. On start up the service 4wd light and all the lights on the selector panel (2hi, auto 4wd 4 hi and 4 low) come on for about 20 sec. The truck seems ok once they go off. It shifts into all the 4
wheel drive options fine. If you take off with the lights on the truck seems to shift from 1st to 3rd or 4th. Once the light goes out after 20 sec every thing is fine. Any ideas for what can trigger this event are greatly appreciated. Thanks Steve

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