Lippert covered the cost on mine, cruiser will tell you they are not under rated. Technically true, they are not under rated. Look under any Cruiser RV product (or other lightweight brand FWIW) 6500-7800# GVWR, you'll probably see dual 3500# axles. Those other models/brands don't have the widespread issues as the Shadow Cruiser. There was some defect with these, I've been told its bad tires, bad steel, improper initial camber or some combination thereof.
I feared the new axles might be to HD for the application and start wearing on the outside. I have just over 800 miles on mine since the swap out. So far (knock on wood) no measurable wear.