When I push the 4 hi or 4 lo button I hear a thumping sound comeing from the passenger side kick panel, the light flashes then kicks back to 2 wd. While in under the blazer dad pushed the button as I tried to rotate the front driveshaft. You can feel it try to lock I. But it just won't. Any help is appreciated.

 

Can you hear the transfer case trying to engage? Also, are you able to switch ranges? Mine is currently doing what yours is doing except perhaps the part about it trying to lock, I don't get any engagement at all. It sounds to me like you may have the encoder motor locked up but hopefully someone else will weigh in on this.

 

I don't know how similar it is to mine but this is what mine needed. It was doing the same thing.
I took my 1994 Chevrolet S10 Blazer Tahoe LT to the Local Chevy Service Department today. It was the Transmission Case Encoder Motor. $142.99 at Advance with a $20 core charge. GM has the part but it is $500 and will take a few days to get there. It cost total to track down the problem and put the new motor on $258.00 So if you are having this problem and it is not your switch, vacuum lines or TCCM then it is the Transfer Case Encoder Motor. Hope this helps someone else that has this problem

 

https://blazerforum.com/forum/tech-a...ill-help-7180/