The symptom mentioned doesn't sound like bump steer and wouldn't have been experienced anyway under the conditions mentioned, I don't think. Am I wrong? I understand those angles could be improved and that it's possible bump steer is just waiting to happen when that front end traversal increases off-road. I'm just suggesting that this might fix a different problem than the one mentioned.
My limited experience is with my 4"+ lift, where the rig wouldn't center up at all after a steering input. Move the wheel right or left and let go, the rig would stay on that new course. I'm thinking you might feel this symptom as one of needing more correction at the wheel. I looked at caster and it was definitely at the low end of the recommended range--and that's for a stock vehicle.
Anyway, did the alignment guys give you a printout that includes before and after caster? I added in enough to take me to the maximum (or even a degree over) recommended in the service manual. This eliminated all problems up front (along with toe-in correction, of course).
As far as 'heavy duty' stuff, my opinion is that the geometry has to be corrected in any case--and you can do that with the gear you have. All heavy duty does is help ensure that the geometry to select stays that way--it could just as well be wrong or right with light weight or heavy duty.
I'm definitely not an expert, so if I'm missing something please let me know. It's just that the symptom reported seems like more of a 'steady state' problem, whereas the tb/dl angle issue is a dynamic one--to me. I think I also understand that, just because something "worked for me" doesn't make it right, so I'm all ears.
