Reddit mentions of Ergon GP2 Front and Rear Twist Grips, Small

Ok, so it seems to be ulnar nerve compression which would primarily result in sensory deficits (aka numbness, tingling, stinging) to the ring finger and pinky finger. It would also result in motor deficits (difficulty moving) some of the intrinsic muscles of the hand including lumbricals 3-4, the hypothenar muscles, and PADS and DABS. All of those things together would make most of your hand ache.

So, management is avoiding compressing the ulnar nerve (duh), but what does that actually mean in practice? The best thing to do is to hold your hands in a more neutral position while riding. The neutral position for the hands/wrists is holding your hands in front of you with the thumbs pointed up (like riding on the hoods on a drop-bar bike or this), which is problematic for a ATB/MTN bike with straight horizontal bars. For your handlebars, you should try to get some Ergo Grips which have a better shape for reducing ulnar nerve compression than straight round grips, but ideally you would add some grips with bar ends like these Ergon GP2s which would allow you to use a neutral hand position and avoid compressing the ulnar nerve. Padded gloves may also help, and surgery is available if this becomes a terribly severe chronic issue.

I don't know what kind of riding you do on your Haro 29er, but hopefully these suggestions are feasible, comfortable, and won't limit your riding. Let me know if you have any questions or if I was unclear.

Also, I just checked my notes from the fall; the "definitive" test for ulnar nerve compression is a clawing of digits 4 (ring finger) and 5 (pinky finger) when trying to make a fist.