My 1999 blazer seems to try and engage 4wd on the passenger side only above 60 mph. It begins to roar and as I slow down it gets louder until am below 5 mph and then it disengages. It will go back to normal unitl I am over 60 mph and then it happens again. Any help why it is doing this is greatly appreciated. The switch on the dash stays on 2 Hi during this process.

 

Those symptoms are much more typical of a bad hub bearing than a 4WD problem. Have you checked the hubs?

Other than the roaring noise, is there any other indication that the axle is trying to engage?

 

Not that I can tell. i was planning on replacing the hub. Just never seen one act that way. I didnt think a bad hub bearing would come and go like that.