Try unloading and loading the front end a little with a jack and see if anything is shifting or loose.
I'd bet you are getting bump steer...Idk if it has anything to do with your issue but you should install this steering-
To find it, you will need to move the height between the axle and the frame, and look for movement in the tires/steering wheel. Set the front axle on Jackstands so the tires are just off the ground. Center a jack under the front bumper and lift/lower the body relative to the axle. Watch to see if the tires turn as the frame/axle height changes.
My bet is that acceleration is lifting the front, causing a steer to the left, and deceleration is compressing the front causing the steer to the right.