I have a 1999 Blazer with 4.3 and 305,000kms.
Just recently, after driving for more than 5-10 minutes the Service 4wd idiot light comes on. If I restart the engine the lamp clears and about 10 minutes later it comes back on.

This leads me to believe that this is not "yet" a hard fault.

I am in 2WD when I start and never attempt to engage 4wd.
Any ideas on what kinds of sensors and faults can trigger this lamp.
I do not hear any abnormal sounds when driving.
I am going to start by checking fluid levels (this is a pain ... crawl under the car, and yes I do use jack stands), but any other advice is welcomed.

Thanks

P.S. Another friggin wheel bearing is starting to go ... I am just starting to hear the telltale sound. I wish they would be better engineered ... I think this is about the 7th one I've had to change, I should have kept better count.

Your Blazer needs to be scanned with a scanner that'll read ABS codes. In the meanwhile, check the electrical wires going to the front hubs for broken insulation.