With a Dexter axle, it's usually on a tag with a serial number, the weight
capacity and the Dexter plant where it was made -- Using that info, one can call the plant, speak to The Keeper of Records and get the original specs.
However, I don't know what Burro
used for axles, so it might be a different ball game. There is a LOT of info in the archives about the axle replacement process.
I would try to replace the axle using Dexter's bolt-on weld-on bracket (bracket welds to frame, axle bolts to bracket) so subsequent replacement would be much easier process (axle could get bent or spindle destroyed or whatever) -- If you are having back-road clearance problems now is a good time considering a spacer between bracket and frame to lift the body.