Download link for fxz is in post #5.
You will also need RSDLite. I used this one:
I think you can also use something called "Matt's Utility", but I've never personally used that one.
Anyway, extract the fxz file to your computer, then open the .xml file. There should be 2 lines, one with tz.mbn in it and one with gpt.bin. You will need to remove those lines.
Then boot phone into fastboot mode. With phone off, hold down vol-up and vol-dn at the same time, do not release. Press and hold POWER for a few seconds, then release it. When you get into the bootloader selection menu, release vol buttons. Use vol-dn to go down to Fastboot and vol-up to select it.
Now confirm Fastboot says your phone is UNLOCKED, because this won't work if it's locked. Plug into your PC and make sure you have Moto drivers installed.
Now run RSDLite and select the .xml file where you extracted the fxz to. If you removed the correct lines, it should flash correctly and be on JB when it reboots.
One thing to note, it will do a factory reset. I think there is a line you can add to the xml to keep your data, but I just tried to Google it and couldn't find it.