Overdrive Surge 04' FX4

When in overdrive my 04' F150 FX4 5.4 Triton surges while trying to maintain a steady speed during power delivery to the wheels, it causes a shuttering and bucking effect. If I push the button to turn the O/D off, the RPMs rise and problem goes away. During the problem no CEL illuminates and tach stays constant. Fluid condition & level are okay. (not burnt) I'm completely baffled since there is no code in the ECM to point me in the right direction. I do use the truck to occasionally tow my 5k lb boat & trailer. Thanks in advance for any advice.