Having problems when I go past the 12 fret on low strings

[troy mcclure]

I can't seem to keep my left hand thumb in a natural position and my shoulder and arm seem to cramp. I very rarely play on the E string up that high, but I would like to have the ability to anchor there rather than at the 7 fret on the A string.
I think I keep rather decent position everywhere else on the neck and I play a Jazz neck so the size of the neck is not restrictive.

Any suggestions?