I have a 97 blazer with 190k miles, and here is my problem. Recently, I engaged the 4hi to get out of a muddy parking lot, and upon getting back to the road, I put it back to 2 hi, but I could tell the 4hi was still engaged, because I could still hear the slight extra whine. About a quarter mile down the road, I heard this extremely loud thud and it was back in 2hi. I pulled over and checked under the truck and under the hood but couldn't find anything broken or damaged, so I drove on and its been fine. This has happened twice so far, and I'm wondering what this could be. You'll have to forgive me, I don't really have a lot of experience with 4wheel drive vehicles, I'm more of a car guy. Any thoughts on what might be causing this? Thanks.

 

Check the oil in your front pumpkin and your transfer case. If you take the fill plugs out and shove a finger in there you should have a bit of oil on the tip of your finger. If the oil has never been changed in the transfer case it'd probably be a good idea to change it anyhow. If it has push button with auto don't use anything else but GM AutoTrack fluid. On the back side of the transfer case lower plug drains it, refill (and/or check) through upper and one of those adapters that connects an oil can to plastic hose makes it very helpful. Other possibility is the vacuum actuator cable that engages the front end is getting rusted and "hanging".