i was driving down the interstate this moring doing aobut 65-70 when all of a sudden this really loud noise came from the transfer case , (i didnt push any of the buttons before this) and it would very with speed, so i pulled over and messed around with the buttons to see what was up with it. I started to do again and sure enough at around 20 mph the noise came back so i pulled over and tryed it agian. I put it in 4 high to see what that would do. It made the sound go away , but it did not put it in 4x4 it was still in 2wd so i got it home and messed around with it.
Put it up on jack stands and then put it in drive and in 4 high no sound and it was in 4wd, 4 low worked as well. so took it down and took it for a spin, same problem as before, no 4wd in 4 high( no sound either) in 2wd it made that bad sound, and 4 low was working fine.

any ideas whats wrong with it? searched the forum and didnt have any one else haveing this exact problem but this is the closeest one i could find https://blazerforum.com/m_5443/tm.htm but it didnt help to drive on grass or muddy conditions ( tried that )
thanks for all of your help

 

that sounds almost what was wrong with mine...i needed a new vacuum switch and actuator cable(i think) i cant remember the names exactly but, it sounds like what was wrong with mine, if u take it to a dealer to get fixed expect it to be a few hundred bucks... if its the same thing

 

well it was a vacuum switch gone bad... lucky for me my dad is good at finding out whats wrong with stuff, so we were able to figure it out and fix it, only 39$ for the part at dealer, only place they had it, a lot cheaper than the few hundred for a dealer to fix it

thanks Mcas86