You will have to split the box and recover whatever used to retain the axel
The press the axel out of the wheel, derust the wheel & axel, coat it with some good axel grease or never sieze the put it back on the reassembled axel.
Next time put a pair of scizzor jacks between the wheels & the body and wind them up to put a little force on the axel.
Then hit the axel with an air hammer, when the jacks get loose, do them up a bit tighter and go again with the air hammer till eventually they come off.
It will be a very slow process liberal applications of Kroll or similar usually helps.
