4X4 acting up

I have a 88 S10 blazer that from time to time I hear a growl coming from the right front tire. Shifting into 4high then back to 2 high stops it. My 86 did the same thing. Any thought on how to stop it from trying to engage? The vacuum ?switch? seems to be ok, it just does this when ever it wants. Sometimes 3 times a day other times it won't do it for months.

Thanks for any thoughts,

Delorian