Here are the details so anyone can copy my dolly design.
Post and receiver are Schedule 80 pipe - 1.5" and 2" respectively - with .25" plate and 3/16" bolts for receiver mount. Weld a disk on top of the post tube and weld a raised dot on the center top of that disk (to reduce turning friction). Grease the post to make turning easier. I have less than 10% tongue weight so had to mount the receiver as close as I could to the trailer box to increase the weight on the dolly to get enough traction so the dolly doesn't spin.
Dolly main horizontal member is 2.5" square .120 steel tubing with a .250 plate welded on top to strengthen the post attachment area. Could use thicker tubing and omit plate if desired. Remainder of tubing for dolly is .090 wall - 2x2 (winch supports), 1x2.5 (main vertical tubes), 1.25x1.25 (leg and handle receivers), and 1x1 (everything else). In the last photo below you can see that I ran the winch mounting bolts through a piece of 1/4" plate instead of the washers I was using earlier - the washers weren't strong enough for the load. The longer the handle is the easier it will be to turn the dolly when under load. Ground clearance for the battery end of the dolly is an issue when going from the driveway to the street. Raising this up a bit might be good - I have to use plywood under the dolly to cross the gutter. Don't raise things too much though or you'll have interference with the trailer tongue.
Wheels are 10" diameter from Harbor Freight - part # 30900. Use denatured alcohol or other solvent to clean the waxy film off the bead of the tires to help prevent the tires spinning on the wheels under heavy load. Also pump the tires to at least 30 psi for the same reason. Ignoring this advice will likely result in tearing the valve stems off the tubes (DAMHIK). Spacers between wheels are 2.5" .120 square tubing - make sure the ends of these are really square. Allthread and nuts for wheel assembly is 5/16", grade 7 with double nuts on both ends (McMaster Carr is a great source for this kind of stuff). Axle is 5/8", grade 8 allthread. Valve extenders are poly tubing and valve stems from old bicycle tubes with tie-wire "clamps." This is functional, but could really be improved on. Remove the cores from the tire valve stems if using this method.
Sprockets are used 13/39 atv sprockets. I didn't have the chain that went with the sprockets so purchased roller chain from McMaster Carr. This chain is for wider sprockets, but seems to work just fine and was a ton cheaper than a new ATV chain. Winch is 3,000 lb Harbor Freight winch #95912. Measure length of spool, then cut about 3/8" out of center of spool to allow room for drive sprocket. Use 1.5" .120 wall tubing for slip fit over spool (have to sand away some powder coating to make this fit). Good alignment is important when fabricating the sprocket assembly. I welded both tubes on the sprocket first. Then slid the drive-side half of the spool in and welded the tabs on while it was all clamped together. Use 3/16" flat head bolts to connect spool and sprocket assembly. Idle side of spool is just friction fit - using some JB Weld to solidify this might be an improvement. Make sure spool length is correct when done.
Here's some photos to help visualize the assembly:
[edited by Woodglue 03/18/10 @ 10:02a - stacked pics instead of next to eachother to prevent page bl 
wout]
) it should do much better.
Once you get all the bugs worked out, would you make another one for a ASA member?