I don't know about your State ... here mobile home axles are illegal on the road.
a little over 10 years ago i ran into the same thing. i got three mobile home axles and six good tires out from under a double wide for the total of $100. i had to crawl under and get it all unhooked and out without doing any damage to the landscaping and such, but it was well worth the effort.
anyways, i went to a well-known local trailer dealer to buy a shackle and equalizer kit, and the guy asked me what i was using for axles. when i told him i had mobile home axles he got all huffy and told me they were illegal and that i needed to use commercial ones like he had there for about $1000 (4 tires, 2 axles). he further pointed out two trailers in his yard that he said had been pulled from the road by the police in because they had been cited as unsafe for the axles. both trailers looked like they had been through a twister, underbuilt, misused and abused, and one didn't even have fenders. needless to say, i wasn't buying anything that day.
the next day i went to the state police commercial vehicle enforcement office to get the story straightened out. the police told me that anything is legal for use on the road unless it specifically says: "not for highway use". even the tires for mobile homes that say "for mobile home use only" are perfectly legal to use because they are intended for highway use.
granted, every state has different laws, and they change from year to year, but i suspect that they are fairly similar in your state too. i hate to make any assumptions, but i'm thinking that the trailer dealer may have stretched it a little when telling me those traielrs were taken off the road because of axles. possibly for the bald tires, twisted frame, cracked welds, lack of fenders, and broken lights. many people get a little antsy about legality and safety, but i'd get it straight from the people who enforce the laws before i believe it - maybe you did, and i'm wrong, but i'd be surprised to find they are illegal.