They will rub. They will rub a lot. This will limit your flex because the frame will limit your up travel. This also kills the leverage on the axle and will limit the drop on the opposite tire. It basically just sucks. Avoid this if you can.
To keep youre tires off the frame, you will need more than just the longer axle. My 35"-37" do rub alittle. (But you might be ok depending on how high you're lifted.)
This is with 33"(measured 31.5"), 63" Chevy's(well, 65"), with the stock 1st gen axle, and about 3" suspension lift, on 3.25"' BS rims. Flex dramatically improved when I put the longer axle in(no other mods), this got the tires far enough out that they didn't touch the frame and allowed the axle to do it's thang.
And that is my official "Why the hell won't this tire drop anymore than this?" look.
