The wheel wells are plastic, and don't live forever... and they're quite subject to getting beat to pieces if your steel belted radial throws a belt... Plating the wheel wells will help, but I'd add some aluminum angle as well to tie into the floor and walls of the trailer. The extra pounds and $ will seem cheap later on....
Rather than paying for periodic in-situ running gear balancing, I'd simply get a set of Centramatics
from Inland RV; they balance things automatically, which is nice. This is not a true dynamic balance
but is far better than none at all, which is what happens with most trailers.
To read up on axles, read Andy's link here
Hope this helps -