Wobbling rear wheel/s

[slowboy]

I had my '72 GMC C1500 on jackstands while doing TH350 transmission filter and gasket change. After I got done and put transmission fluid in I started truck and was going through the gears. While doing so I noticed back driver's side wheel was spinning, but was wobbling.
I know the wheel and tire is ok because I had the wheels powder coated and when the tires were remounted they balanced out good.
When I set the truck down on the ground and drove it I didn't notice any kind of rearend shake or shimmy.
Can anyone tell me what/why the wheel was wobbling while on stands and what needs to be done to fix it please?

Thanks in advance y'all!!

Keith [slowboy]