Ok, so a replacement of a fuse seems to have taken care of the 4WD!! (Ridiculous, considering that we paid a mechanic to find the 4WD issue and he couldn't. But anyway..)

As for the heater, the vents all work when changing from lower to top vents but the air is cold and it really doesn't blow hard, even on high. The manual control dial that switches motor speed... I had a 97 Jimmy and the heat controls looked identical to this Blazer. However, when I used the dial to change motor speed, it would click between variances.. This Blazer's dial slides with no clicking. BUT the motor speed does change from low blowing to high blowing so I guess that's not an issue.

I held the RPMs at 2000 for a minute and got hot air from all vents (though the floor vent was blowing very light). Took it for a drive and it seemed to blow hot while driving but then went back to cold when idling. One heater hose gets warm, one does not. So does that indicate a clog somewhere? Should we flush the system?

Oh! And I forgot! When I revved it for a minute, smoke and bad bad smells came from right behind the firewall, near where the hoses go into the dash. The temperature was fine but something obviously got too hot.

EDIT: A good flushing seems to have taken care of the heat for now.