The wheels probably have/had bushings pressed in which slid out of place or wore through. Your application of Harbor Freight stuff to full time, real world working applications is a strenuous test, to say the least.
They do sell 10 in. wheels with real bearings that might be better for your app.
http://www.harborfreight.com/10-inch-pneumatic-tire-30900.html
or 13 inch wheel and tire: even better if you have clearance
http://www.harborfreight.com/13-5-32-inch-x-3-7-16-inch-flatfree-tire-67469.html
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The wheels probably have/had bushings pressed in which slid out of place or wore through. Your application of Harbor Freight stuff to full time, real world working applications is a strenuous test, to say the least.
They do sell 10 in. wheels with real bearings that might be better for your app. http://www.harborfreight.com/10-inch-pneumatic-tire-30900.html or 13 inch wheel and tire: even better if you have clearance http://www.harborfreight.com/13-5-32-inch-x-3-7-16-inch-flatfree-tire-67469.html