I tried to do some digging on our frames being heat treated sometime last year. It turns out they are, as are many factory frames. I have talked to several offroading places and they've all said that it is acceptable to weld to frames for things like sliders. It's more critical that it's welded right than that it's welded at all. You pretty much need to minimize heat soak into the frame so the affected area is as small as possible. TIG welding would be optimal, but the tight spaces may be a problem.