96 s10 4X4 problem

My 4WD will not always activate. I push the button, the light lights but I'm not in 4WD. If I travel a mile further down the road there will be big clunk
and then I will be in 4WD. If I sit at a light and push the button, you can hear the shifter moving so I know that part is working. Other observations: if it does shift to 4Wd, it will be at speeds greater than 30mph. It's intermittant, I've been to the dealer three times and he says everything is fine. Shifting to reverse and than to drive seems to help.
Where do I start looking? I can't get up my driveway without 4WD (thats how I know it's not engaged).
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