The link that Steve provided looks great and the screenshots seem very helpful.
I have a XV6900 and also have it flashed to be GPS enabled. I believe the main reason why we cannot use the stock ROM is because it uses the Radio 1.xx while these cooked roms are based on the original Sprint release that updates the radio to 3.xx. I used DHarvey's methods to Unlock and Update the radio because i felt that his was very easy and straight forward and outline the steps one-by-one.
-Link to DHarvey's Unlocker and REV A./GPS radio update
http://forum.ppcgeeks.com/showthread.php?t=24979
*this has to be done first to update GPS radio, but its technically a Sprint ROM which could work on your phone, but looks pretty plain... plus the phone codes don't match
-Flash you own custom rom of your choice. Honestly, once you go custom, you never go back. They run faster, are alot cleaner and usually the guys that cooked these up have a good selection of apps and software preloaded into their ROMs. I've used DHarvey's and Whosdaman's roms, both are great.