You need to find an old alignment guy. Not late 40s old, I'm talking about a chain-smoking, bent in half grey old man who thinks an alignment machine is some rebar under a metal plate and two pieces of string.
Seriously though, aligning a TTB front end doesn't have a lot in common with these rack and pinion systems used on everything these days.
Frame angle has to be measured accurately. I've found that 1/2 degree of frame angle is enough to mess it up.
Second, camber and caster must be set correctly. They can both induce a pull, but the factory specs are set up to make the vehicle naturally go to the right, so that if something happens to cause you to loose control of the steering, like driver incapacitation, the vehicle goes off the road instead of into on-coming traffic.
Third, when setting the toe don't use a steering wheel holder, leave it free, get one wheel to zero degrees toe with the steering wheel straight, then move the other wheel to zero degrees toe while bumping that first one back to zero with your hand if it moves. I've had the steering wheel holders screw me over many times on these trucks, and that zeroing method has worked every time.
You can also try flopping the front tires across the truck and see if it starts pulling to the left. Tires can cause a nasty pull that all the alignment in the world will never fix.