95 Blazer LT 4x4 disengage problem

1995 LT auto w/ elec 4wd, 4.3L 'W' with 100K. Fortunately this is my wife's car and we don't need 4wd often. Been 1+ yrs since 4wd used until last week. Went in okay and seemed to go out okay (2wd light burnt out...) until driving down the highway today only to see a plume of smoke out the rear. ATF comming out the vent tube (attacted to the fill tube) running on the manifold. The engagement of 4wd was flakey - blinking lights, then both 4Hi and 4Lo would come on when 4Lo selected. After checking the vacuume hoses and everything okay, I found that the front Diff was still engaged - HOWEVER, when I pulled the 4-port vacuume connector off the top of the xfer case I heard the vac. release as well as the fron diff. disengage. I also disconnected the battery ~10 min. (another previous thread) The 4wd seems to engage properly now but still have an issue with the vacuume (eg. won't fully release the front diff.). Any help or thoughts would be greatly appreciated. Happy New Year!

Common problem, bad vacuum switch on top of the transfer case.

Replaced vacuum switch (Autozone #49315) $15 and problem resolved! When I pulled the defective switch it was stuck open. I did have to pull the neg. terminal again and perform a reset.

Thank you Ricksza! Scored some serious points with the Mrs.!!!