I was about to start drilling the one cabinet drawer where everyone was in agreement and I noticed the drawer itself is not level. The drawer face sits lower on the left side. I can raise it up by hand to a level position.
The rails are slightly off level as well. The left rail has a lot of wiggle in it, where as the right is a bit firmer. It looks like the back of the rail has an adjustment index. The grey plastic bit in the back.
Any idea how to tighten things up and get it level before I drill?
That's off by a LOT. You sure the drawer was resting squarely on
both undermount slides? It looks to me like it wasn't seated on one.
Undermount slide systems all have micro adjustments for drawer leveling, but I'd need to see your system up close to know what yours uses. If you can find a part number, google it, and you'll probably find the manual. I've got various generations of Bluhm slides, and they have either knurled nut adjustments, or a plastic cam with click stops. I just can't quite make out your picture well enough.
Forget the back. These should have a track on the back and fit loosely. It allows the drawer to pivot horizontally a little, but is self correcting when closed. If you push on one side of the drawer front while pulling on the other, you should be able to rack everything, causing the slides to shift in their tracks, and when you close the drawer and push it to its stops, everything will realign. It's a REALLY nice system.