D37 final drive problem

My d37 stopped pulling on the left track, when we pulled the cover off the sprocket hub we found that someone had welded the nut to the axel and hub, I assume the axel was stripped and this is how they repaired it, How expensive is it to replace the axel and hub. This is a good running and operating dozer and this is the only problem I have had with it in 2 years.