There are allot of shops that can and are willing to do the frame. What there is not allot of shops that can or willing to strip the body off the frame and then replace the body back on the new frame. Do you have the ability and place to remove and replace the body?
Sometimes these things are a house of cards. A trailer with a badly rotted frame is likely to need more work than just a frame replacement.
Welding shops like to build steel structures its what they live for. Most welding shops wont even want to paint
the new frame when its done.
Step 1 remove the body shell.
Step 2 order the new axle
Step 3 once the axle
comes in take the new axle
and old frame to your local trailer builders and weld shop and ask for help.
Don't bother calling these shops just to hear a big we don't do that. Make them see the problem and deny or quote face to face. You will eventually find a place to help you. There's always welding night classes you can take to get it done.