Could be that you're lacking positive caster. Not something really discussed on a Porsche or car forum but definitely something that all truck/4wd owners have to deal with as they lift the front and rotate the axle forward for pinion angle. Eventually you will get to the point where the wheel won't return. Are these shops checking for caster? Its possible that its only off a little...doesn't take much to ruin the steering.
Wheel Caster