I have a 97 s-10 4wd reg cab shortbed pickup. When you try to shift into 4wd high or low it acts like it works but no power from the front wheels. Sometimes 2wd light will flash. It just spins the rear tires until you are buried. Ever once in a blue moon it will go into 4wd like nothing is wrong. I have replaced the actuator under the battery area and checked my lines. The switch instructions say it will flash and go to 2wd if it is bad, but it doesn't do that so I didn't use the new switch. What else could it be? The rear 4wd motor?

Is the front drive shaft locking in?
If so check right front axel for locking in.

http://www.automotiveforums.com/vbulletin/showthread.php?t=572914

Lot of 4W post on here.
Do a search on some of them.