HRL has pointed you to the correct things to look at. I have a similar issue with a stock truck(tire scrub in low speed turns to the right) It is a worn/loose king pin. That lets the left front wheel move and the toe in goes to toe out. Truck steers and tracks straight. It took a lot of looking to find the left king pin moves at the top just a bit. Not what I wanted after I had replaced the bushings(my bad for not noticing the top bushing was easy to push in). The thing is, very few folks understand the Geometry and issues. With the changes you have to your Truck, Consider every thing. Do you have a stager? Different Camber on one side or the other? Any loose pivot points? Equal toe in on both front wheels? That one is tough to measure. Any difference in Caster from side to side? All of these things matter. Lowered? Scrub angle? And centered in the steering box? Lots to look at.
