You can buy a left side hub that clamps to the axle instead of using the threaded stud. Those are available from IRS and CRC. There might be some other companies making these hubs, but I am not aware who they are. But the clamping hub does not depend on the threaded stud for locking.
I would suggest you check with Ultimate about who to contact about replacement of the axle. If it is an Associated axle, and I am pretty sure it is, you can check with them about a replacement through a waranty.