The only thing changed was the prop. The skeg is the permanent fin on the bottom of the lower unit... correct? Do you have an aluminum alloy fin right above the prop? Looks like this? You can adjust it one way or the other to straighten out steering pull at high speed.
TRIM TAB ADJUSTMENT
On larger Outboard motors, there is a steering trim tab located on the bottom of the cavitation plate, just aft of the propeller. It is adjustable and has been installed on the engine to correct for propeller torque.
Adjustment of the trim tab should be done after you've gone through the steps of proper tilt adjustment.
The boat should be operated with a normal load on board in a straight line. Incorrect trim tab adjustment will cause the boat to turn to the left or to the right when your hand are removed from the steering wheel. Be careful when removing your hands from the steering wheel; a tilt tab incorrectly positioned may cause the boat to turn violently.
To adjust the trim tab, loosen the securing bolts, as per the engine manufacturer's instructions, and move the trim tab in the direction the boat pulls. For example, if the boat pulls to starboard, move the trailing edge of the trim tab to starboard. Make these adjustments in small increments and test the results. The object is to create a "hands off straight line" steering condition for the normal operating speed. It is not possible to eliminate steering pull throughout the engine speed range.
Remember, tilt adjustment may affect steering torque as much as the trim tab. With Outboard engines equipped with power trim this becomes particularly important. When under way, experiment with trim settings to neutralize steering pull.