I decided to hold off on the expense of an enclosed trailer and did a major overhaul of my current 18' Anderson closed deck steel trailer. Basically replaced all the running gear. New springs, hubs+bearings, brakes, and all connection hardware. Cost a little less than $500 from etrailer (free shipping too!) for everything. Once I got under there and started taking things apart, it was scary to find that 3 of the 4 leaf springs had broken the center pin and one had actually lost the bottom 2 springs (leaves?). That would explain the thing sitting a little lower on that side, a bit of extra sway, and a LOT of noise. The adjusters on the brakes were completely rusted solid, so they weren't adjusting (supposed to auto-adjust when backing up) and the less than stellar braking. On the plus side everything else looked surprising good. A lot less rust than I expected, and all the hardware loosened up without the use of a grinder, cutter, hammer, or giant pry bar. Next step will be to give it a good coating of new paint. Thinking of doing something like a bed liner for the surface, and just rattle can the rest.
Anyway, the thing is now so smooth, quiet, and brakes incredibly well. Using the brake controller it can easily lock the wheels when unloaded and does will slow down the whole vehicles when loaded. So for about $550 it's basically a new trailer. Should be good for a few more years of good service.