I had my own custom JB Rom based off of the Official JB release from Sprint and am now running my own custom JB Rom based off of AT&T JB Release and am having no issues with GPS. I actually never used Odin to flash the official release for AT&T. I recommend factory reset, format system, clear cache and davlik before flashing. After flashing Rom reboot phone setup your phone then reboot back into recovery and flash the LK3 modem to see if it takes. Problem with modems is sometimes you need to flash it a couple of times. Also I recommend using TWRP since it seems to be a bit more stable for our phones when flashing things lately...
DX
I never flashed the Odin version either. But i did use it and mobile odin to update my modem
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