The steering rack mount could be worn out, allowing the rack to move. The issue sometimes gets compounded by having an alignment performed when the rack is out of alignment, meaning you'll spend more time screwed up than set up properly. A good fix is using the PreferredChassisFab (member here, PM him or look in his area in the sponsor section) steering rack bracket. If your rack is moving then your wheels and tires are not moving properly when you turn the wheel.
Other issues could be worn ball joints. When turning, if the ball joints are worn the wheels again move out of alignment. Upper ball joints must be replaced as part of the upper control arm. Lower ball joints can be replaced independently, like with this one:
http://www.amazon.com/gp/product/B007QBVKXW/ref=wms_ohs_product?ie=UTF8&psc=1
Less likely is worn bushings on either the upper or lower control arms. Again, the uppers must be replaced as a unit but the lower bushings can be replaced independently.
I noticed in your signature that you are using the 1" TD bumpstops - is that correct, you're not using the 1/2" ones? If so, that suggests your front end is cranked really high, maybe 24" or more from center of the wheel hub to the bottom of the fender lip. If so (and I'm speaking from expensive experience) you're creating a lot of wear and tear on your front end from the steering rack (bushings, inner tie rods, outer tie rod ends), ball joints, CV joints. Most members here use the 1/2" bump stops and keep the truck at 23.5" or below.
Thinking about it, you should check your CVs as well and, at a minimum, make sure the boots are not torn and grease isn't flung out.
