B737-300 DEVELOPED A FUEL IMBALANCE AFTER CROSSFEEDING. THE IMBALANCE INCREASED AND THE POSITION OF THE CROSSFEED VALVE COULD NOT BE DETERMINED. FLT CREW DECLARED AN EMER AND DIVERTED.

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AT FL350; WE INITIATED A NORMAL XFEED EVENT. AFTER A FEW MINS; WE GOT INTO AN AREA OF MODERATE TURB. XFEED WAS COMPLETE; I TURNED THE XFEED VALVE TO CLOSE AND SIMULTANEOUSLY INSTRUCTED MY FO TO TALK TO THE FLT ATTENDANTS AND PAX ABOUT THE TURB AND HAVE THEM ALL BE SEATED. I SUBSEQUENTLY REQUESTED LOWER FROM ATC FOR A BETTER RIDE. ONCE WE STARTED THE DSCNT TO FL310; WE NOTICED THE XFEED VALVE LIGHT WAS ILLUMINATED. WE COULD NOT TELL IF IT WAS BRIGHT OR DIM. ALL MAIN FUEL TANK PUMPS WERE ON AT THIS TIME. WE DOUBLE-CHKED BOTH FUEL QUANTITY GAUGES AND AT THAT TIME NOTICED NO IMBAL. WE PROCEEDED WITH THE QRH. AFTER REVIEWING THE QRH; 'XFEED VALVE INOP' CHKLIST IN ITS ENTIRETY; WE COULD NOT GUARANTEE; WITH CERTAINTY; THE ACTUAL POS OF THE VALVE. WE COMMUNICATED EXTENSIVELY WITH DISPATCH THROUGH ACARS AND BROUGHT MAINT INTO THE LOOP AS WELL. WE NOTICED AN IMBAL STARTING TO OCCUR WITH EQUAL THROTTLE POS. CIRCUIT BREAKERS CHKED IN AND NORMAL. WE ATTEMPTED DIFFERENTIAL THRUST FOR A SHORT PERIOD OF TIME. THIS DID NOT CORRECT THE IMBAL. THE 'HIGH-SIDE' GAUGE FUEL QUANTITY DID NOT SHOW ANY MOVEMENT THROUGH THE ENTIRE EVENT. AFTER CONFERRING WITH DISPATCH; WE DECIDED TO DIVERT TO ZZZ. WE DECLARED AN EMER AS A PRECAUTIONARY MEASURE AND LANDED UNEVENTFULLY. UPON LNDG WE HAD AN IMBAL OF BTWN 2000 AND 3000 LBS. ACFT HANDLED NORMALLY IN THE AIR AND ON THE GND. LOGBOOK ENTRY WAS MADE AND ACFT HANDED OVER TO MAINT. IT'S HARD TO SAY. IN HINDSIGHT; IF WE COULD GUARANTEE THE VALVE WAS IN FACT OPEN; I WOULD HAVE CROSSFED OUT OF THE HVY TANK. NOT KNOWING THE ACTUAL POS OF THE VALVE; I WAS RELUCTANT TO TURN OFF ANY FUEL PUMPS AND CAUSE FURTHER PROBS. THE FACT THAT THE HIGH SIDE FUEL GAUGE WAS NOT MOVING WAS ALSO A CONCERN FOR FURTHER COMPLICATION.