I have a 1992 blazer tahoe, 4.3 vortec, pushbutton 4wd. I pushed the 4wd button (while in park), when I shifted into reverse, A very loud grinding noise came from underneath. I put it back in park, cycled the 4wd button off and on, then tried reverse again, with the same loud grinding noise. I took it out of 4wd and drove it no problem. I can put it in 4wd now with no grinding, but I don't believe the 4wd is actually engaged because there is no difference in grip and the front tires won't spin. I can hear the 4wd engaging when I push the button on and off.
Any ideas where to check first?

 

I'm pretty sure that you have a vacuum switch on the right side (near the top) of your transfer case. It may be that a vacuum hose has come off.
The vacuum source is from a vacuum tank near the brake booster. The vacuum then goes down to the transfer case switch, then to a vacuum actuator under the battery tray. Check for vacuum leaks.

This may also be the cause of the engine running rough ( I also read your other post)

 

ptcoins... Have you checked the vacuum supply to the front axle actuator?