My thoughts exactly…or some sort of nerve impingement causes tingling…
Try Listerine mouth wash - comb a little down the tail - has worked on some horses I’ve known with the constant ‘tail rubbing’
Good Luck - it’s very frustrating, I know !
And did you check her for a “bean” in her urethra folds? I was cleaning my mare today and probing around in there and sure enough found a nasty sharp maybe 1/4" “bean”.
I’d pick up some Equiderma lotion and see if that works. Have you talked to your vet? If it’s allergies, you might try feeding Benadryl to see if there’s any difference.