URGENT: 4wd LO engages on its own

I have an 01 4x4 Blazer (trailblazer package).  I see I've had most of the problems everyone else has.  I think or at least thought my vacuum system was going bad because I couldn't engage my 4wd all winter but haven't had time to fix it.  But for the now urgent problem.  My 4x4 LOW is engaging itself as I drive. Whenever I shift from reverse to drivee it engages and as I drive it is flashing like it tries to engage it.  Stopping makes it engage as well.  The problem is that once in 4x4 LO it doesn't want to then go back to 2wd.  Any suggestions as to a repair? 

Also could anyone clue me in as to where to check the vacuum system and I heard a guy talking at the gas station about a similar problem with the 4x$ due to corrosion. Ammusingly he was talking about it the day before it happened to me.