I have no idea what could be related if it only happens in overdrive. I'm mostly retarded when it comes to automatics. My friend had a problem with a jeep grand cheerokee doing the same thing. I guess he ran it down a mountain pass in 2nd gear and it was never the same. lol. In overdrive with the torque converter locked it bucked and grabbed, torque converter didn't seem to want to stay locked...
But if it has really been sitting so much I would he changing a few things anyway. Related or not, fuel filter could easily be bad. Sitting so long allows condensation to form in the tank and pool at the bottom. It's best to keep fuel tanks full to keep that from happening.
What's the ATF level look like? Any idea when it was flushed last?
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT