You need to take the truck to a good alignment shop and get it checked out. Without a reference on what the caster angle is now, there's no way to tell whether it needs more or less caster. If there has been a radical change in ride height, which is the reason most people install non-stock springs, the alignment could be off in a big way. If the steering does not try to return to center after a turn, you probably need more positive caster angle, but that's not a substitute for proper wheel alignment.
Jerry
