98 Blazer 4WD keeps engaging?!?

I have a 98 Blaxer 4wd that keeps engaging in 4 HI. When I put it into gear, about every 10 seconds, it keeps "engaging". 4WD worked fine before and it still works but need help on what is causing it to keep doing it. Like I said, you will be driving in 4HI, and it sounds like it keeps re-engaging. Any ideas or thoughts?! Thank you soo much for your help!

I'm sure there are others who can help you better with this, but there's a post from a few days ago with a similar problem.
http://www.automotiveforums.com/vbulletin/showthread.php?t=1055873
I'm guessing you have a vacuum leak or a stuck vacuum switch. I've read several posts about bad vacuum switches on top of the transfer case and/or the switch under the battery holder.