Scheduled maintenance on a Grand Marquis.
 

My '99 Grand Marquis has just 40,000 miles, and the tranny runs just fine, but the fluid looks alittle dull in color, so I'm getting a fluid exchange soon. Should I get a fluid change on the differential, as well? (And maybe flush the radiator while I'm at it?) Thanks in advance.

Yes. Yes. And yes. Regardless of the low mileage on the vehicle you still need to take into account the age. For example that's why for so many years oil changes would be recommended every 3 months or 3,000 miles (with technology such as synthetic oils mileage intervals have increased quite a bit) in terms of maintenance / preventive maintenance, the age of fluids is just as important as the amount of use those same fluids. I would also recommend a reputable brand fuel system cleaner as well.