im so tired of this car, the problem was originally it would only shift into 4 low and 2hi and not 4 hi. I swapped the battery and 4hi would engage and stay engaged immediatly. But after putting it back into 4low and 2hi it would no longer go into 4hi. except now while im driving at any speed in 2hi it will engage the vacuum pump and engage the front differential randomly and make absoluly atrocious noises. i disconnected the vacuum line and it works disengaged in 2 hi just fine. has anyone on the internet had the same issue? is this a computer issue that i should burn this car over? or is it the vacuum pump itself? there is no CEL. EDIT: 1999 blazer 4.3 with the standard 3 selector switch with no auto 4x4.

 

What year, engine and I'm assuming 3-button selector switch? Are you saying selector stays in 2-Hi but will engage front axle without moving out of 2-Hi? If so, I might first suspect vacuum switch mounted in transfer case. Maybe remove and inspect it. If the ball in the switch moves in very easily (with little to no resistance), could be a broken spring in the switch.

 

my bad its a 1999 with the standard 3 selector switch, no auto 4x4. Ill check that tomorrow.