I know in Texas, if you built trailers, as my grandfather and I had in the past, if you wanted to use it commecially, you had to have it tested for DOT compliance. If the DOT inspector looked at it and passed it, you were good. Many companies that build them commecially have them tested by an independent laboratory. That relieves the state and builder of liablity, saying it was tested by a third party. The same goes for the truck frames, to a certain degree. Private vehicles are not as strict, but if it breaks without certified testing, look out for the buzzards... er, ah, lawyers... . On shop built trailers, weight registration is whatever you say it is, as long as you're not overaxle. Commercial truck modifications can be done by anyone, but class weights can only go so high, like Light Duty pickups, class 5, 6, 7, 8 trucks, up to 128,000, then you go to permitted vehicles. Titleing a pickup for GVW over 24,000 is frowned upon... ...

Dealer titleing limits the trucks GVW for the frame and axles, so you have to refile for title, which is a major PITA, and they do it on purpose, and tax the heck out of you, too.