Hi, I'm moving some electrical around for a kitchen remodel and I wonder if
it's ok to bury a steel junction box behind the sheetrock.

No, but you can flush-mount a single or double-gang switch box and put
a blank cover on it. That or install an access panel.

Better yet, question why you need a j-box in that location. If
possible, rough out the right length wire(s) to/from their termination
points. More work and might be a PITA, but no oddly placed j-box to
worry about.