I have one of these trailers:
http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_4887.jpg

The little wheels at the bottom broke the first time I tried to store
the trailer, so I replaced them with much larger swivel casters with brakes.

But the steel bar they attach to really isn't strong enough once you
have plywood and a spare tire attached, so they are bending, and the
casters are breaking. The trailer is stored for very long periods of
time on these wheels, in fact today is probably the first time I used
the trailer in four years (went to buy used heavy doors for a counter top).

I need to figure out what to do to replace the wheels. I don't care if I
have to remove the wheels before unfolding the trailer (I have to remove
two of the replacement casters now anyway to unfold it).

Right now, it's very unstable when you fold it and stand it up,
especially since the tongue is still unfolded when you first stand it
up, then you have to carefully fold the tongue down without having the
trailer fall or the tongue fall on your head.

Since the whole trailer only cost me about $225 when I bought it, I'm
trying to not spend $200 on a solution.

I'm thinking I may not need swivel wheels since I only push it
perpendicular to the regular wheels. Maybe pneumatic tires from Harbor
Freight, attached to a 4"x4" pressure treated board.

Any other ideas?