my Mavic had a disagreement with a tree a few days ago and fell about 15 feet. Now I keep getting a GIMBLE VIBRATION ABNORMAL warning on the screen. Is there anything I can look for or diagnose before I have to send it back to DJI?

Have you tried to see if there was no binds ?????????

check the rubbers that hold up the plate.

I  had a similar incident just yesterday, and got the same warning.  My gimbal looks fine, and seems to function properly.  l reviewed the flight records though, and see that the gimbal warning does not appear on screen during flight playback.  Does your flight records show that message?  

I had the same issue after I had a disagreement with a tree on the elevation of that highest branch.  For me it was the rubber bands that mount the upper dampening pad (that the gimbal mounts to) to the lower dampening pad.  I was unable to find a place that just sold the rubber bands, or even just the lower dampening pad.  But you can buy both together for about $15 on Amazon.  It will take a tare down to replace but at least you don't have to deal with sending it to DJI.  It could also be possible that one of your props is bent.  I tested mine by taking all the props of then putting one on at a time and holding the drone and giving more power as I watched the gimbal vibration.  Unfortunately for me it was not the props.  If you are getting the abnormal vibration alert you are also probably seeing a lot of the jellowing effect in the videos.  The new dampening pad fixed both issues for me.  Good Luck!

I fixed my gimbal problem after I discovered that the gimbal plate on my Mavic had become dislodged from the gimbal hook.  This hook is basically an "L" bracket that's mounted to what I believe is called the absorbing board.  I carefully manipulated the  gimbal plate back under the hook.  I flew a battery and did not receive any more "abnormal" warnings.  I hope that your problem can be resolved with this remedy.
(edit)  For clarity, I did not disassemble the gimal mount from the Mavic...The image below was something I found on the net and posted, hopefully to illustrate my aforementioned explanation.