tranny not shifting into overdrive
 

My 2003 is not shifting into high gear, even at 50 mph +. Ihave not had any issue with this truck before. I have it serviced regularly and have only towed with it twicw ((6,000) plus lbs. It is shifting fine from 1st--2nd. Fluid looks good and smells good , no burn smell or anything. Truck has 90,000 mile.