• Rear tires are the first thing to check. When these bikes were new they all ran 4.00-18 sizes, a squared-off rear tire. Then tires converted to metric. Their height was reduced and their shape was also changed to be more rounded. That means that you can't simply convert 4 inches to 101.6mm and expect anything with those width dimensions to work. 

Check the right side of the tire for signs of rubbing, and the inside of the drive shaft housing for a polished area devoid of paint. 120/xx tires have the right "look" and have the correct height to operate the center stand, but unfortunately are too wide for Airheads without the inclusion of a spacer.

It's most proabably the rubbing on the drive shaft housing which you are experiencing. Add a steel washer, approximately 0.09" thick, to the rear axel on the RH side of the wheel. This washer should hug the axel and not touch anything else on the OD. It will simply come to rest between the wheel and final drive housing, and push the entire rear wheel slightly to the Left. You must make a mental note of its presence and remember to replace it when working on the rear wheel, because it will not appear on any parts list.

Hope this helps.

If you loosen the axle nut, the wheel spins freely?  This says to me that the bearing stack inside the hub is not shimmed correctly and/or the axle nut is over tightened. Try using a torque wrench and tighten the axle nut to the specified value. According to Snowbum's website, 30 lb/ft should be adequate for the axle nut. (11-13 lb/ft for the pinch bolt after torqueing the axle nut.)

Inside the bearing stack there is a spacer commonly referred to as the "wedding band". This is made in varying thicknesses to accommodate a change in the bearing preload when the axle nut is torqued to spec.