Bob has given good advice above.
My only other suggestion would be to measure the camber side to side to see if one side is bent.
I'd use an inclinometer to measure angle of the square tubing of the axle
on each side, then measure the vertical surface of the wheel hub. The axles are slightly bent in the middle to give a slight camber, so measure each side separately.
You can buy an incliniometer here:
Angle Finder w/ Dial Gauge
Or there is a cheap inclinometer app for IPhone or IPod touch that is handy.
App Store - Clinometer - level and slope finder
If the axle
is bent, you may need to replace it, which is not all bad as you can add brakes