What all is different between a dually rear axle and a single wheel rear axle?

6-8"

Thats all thats different? Its just a little bit narrower?

It's wider. Brakes are different.

brakes, spindles, hubs and flange to flange is different. Axles shafts are about 4" longer due to the hubs/spindles.

Not sure if the spring plates are different or not.

Overall diameter of tube is different too

I can measure I have a dually and a single wheel sitting in my driveway :D.