2002 Kia Sedona Shifting Problem

Hi, Developed a problem w/our 2002 Kia Sedona which when it hits 3rd gear to overdrive, seems not to be able to shift into/out of it properly, and a noticeable but low-level 'hum' type noise is evident that almost sounds like road noise. Tach is observed shifting normally during 1st to 3rd gear transition and levels off at 2000RPM, but at the point this problem surfaces, even though my foot remains on the gas at a steady speed, tach is observed to drop ~ another 500RPM. Anybody have this problem before? Perhaps it's speed sensor related? TIA!