i've moved the CG toward front this time to see if it would reduce the sway, which it didn't. We stopped and moved some things around, and it still didn't change! I've also switched from pulling it on a long bed ford to a hummer, same reaction.
After a few calls around to the trailer dealers, they seem to think the tires are the next probable option.
The biggest question is how in the heck to figure out which tire to get?
Carlisle tires sound like total junk, so then I looked at goodyear and firestone, which had horrid ratings from customers too. That leaves the shipshape trailer tires in a radial (I think).