If your steering shaft and sector shaft are solid (no play in them) and your tie rods ends are good, and your toe-in is set to spec, , then the problem will most likely be your caster angle.
See thread linked in the 2nd post, above.
Get an angle finder, or go to an alignment shop and determine your current caster angle. Then order steel shims to result in caster angle of 5° to 7° positive caster.
When I installed a '74 D30 and 2.5" suspension lift into my '71 CJ5, I could not keep the Jeep between the lines over 35 mph. An alignment shop told me my caster was a 1° positive. I ordered and installed 5° steel shims. After that it tracked perfectly, even at 75 mph...