AN ACR B737 FLC DISCOVERS THAT THEY ARE UNABLE TO CORRECT A FUEL IMBALANCE PROB. THE ARTCC RADAR CTLR CLRED THEM DIRECTLY TO THEIR DEST WHERE THEY LANDED SAFELY.

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I WAS THE PF ON A ROUTINE FAA PART 121 FLT FROM PIT TO MDW; ON THE FIRST LEG OF A PLANNED 2 LEG DAY. DURING CLBOUT; THE CAPT MENTIONED HE WAS GOING TO XFEED IN ORDER TO BAL FUEL BTWN THE WING TANKS. AS WE WERE IN CLOUDS (1200 FT OVCST IN LIGHT RAIN AT PIT); MY ATTN WAS FOCUSED PRIMARILY ON FLYING; NAV; AND WATCHING FOR ICING CONDITIONS. SHORTLY AFTER LEVELOFF; THE CAPT INDICATED THE XFEEDING OP SEEMED TO BE PROCEEDING SLOWER THAN NORMAL. AT THIS TIME; THE FUEL BAL WAS WITHIN LIMITS (AND THERE ARE ALTERNATE MEANS OF BALANCING THE FUEL SUPPLY). APPROX 50 MINS INTO THE FLT; WITH THE AUTOPLT ON; THE CAPT CONCLUDED THAT HE WAS PERFORMING THE XFEEDING PROC CORRECTLY; BUT THE IMBALANCE WAS INCREASING. HE USED THE QRH CHKLIST; WHICH CALLED FOR USING DIFFERENTIAL THRUST. THIS PROC DID NOT HELP THE SIT; SO WE REQUESTED VECTORS DIRECT TO MDW (WE WERE JUST BEGINNING THE STAR AT THIS TIME); THEREBY DECLARING AN EMER. THE CAPT TOOK OVER AS PF; AND I ASSUMED PNF DUTIES. AN UNEVENTFUL LNDG WAS MADE AT MDW. WX WAS A MINOR FACTOR. ALTHOUGH MY ATTN WAS DIRECTED TO FLYING DUTIES DUE TO THE CLBOUT CONDITIONS; IT WAS PREDOMINATELY VFR ON TOP. AT THAT POINT WHEN I WAS ABLE TO DEVOTE MORE ATTN TO THE PROB; THE CAPT HAD ESSENTIALLY ALREADY PERFORMED ALL THE APPROPRIATE ACTIONS. THERE IS A LACK OF GUIDANCE IN THE B737-200 MANUAL REGARDING FUEL IMBALANCE LIMITS. THE MANUAL LISTS SEVERAL LIMITS; BUT NOT WHAT THE CONSEQUENCES OR APPROPRIATE ACTIONS ARE IF THOSE LIMITS ARE EXCEEDED. IT DOES NOT ADDRESS WHETHER EXCEEDING THOSE LIMITS CONSTITUTES A NEED FOR AN IMMEDIATE EMER LNDG; OR JUST A MINOR MAINT ADJUSTMENT. ALSO; THE MANUAL FAILS TO MENTION THAT THE UNDERLYING CAUSE OF THE FUEL IMBALANCE MAY ALSO CAUSE THE FUEL IN A PARTICULAR TANK TO BE UNUSABLE; EFFECTIVELY CUTTING THE RANGE; ENDURANCE; AND FUEL SUPPLY IN HALF. THIS WOULD COMPOUND THE SIT AT A TIME WHEN THE MAIN CONCERN MIGHT BE AERODYNAMIC CTL DUE TO A CTR OF GRAVITY IMBALANCE.