I rooted my phone with Motofail and then installed the OTA update to 2.3.6. And no, I don't have a Voodoo Rootkeeper backup because I simply forgot to do that. Dumb I know.

The superuser app is still in the app drawer, but cannot grant permissions. Is there any way I can reroot, or do I have to SBF and start over from scratch?

Thank you.

Do you have a copy of pete's root tools? I know it's not for the Droid 4 but I used it when I ran into the same problem before the fastboot files came out. The unroot worked to remove my superuser and then I just rerooted with the D4 tool (motofail).
Get it here if you don't have it: https://www.dropbox.com/s/qtuu2eyzwnudyyk/PetesMotorolaRootTools_v1.07.zip