I understand there is balance equipment that is able to balance under a simulated load. I've never seen one. If you have any old tire shops around, they may have an 'on the car' spin balancer.
When I first started out busting tires, the only option to bubble balance was a spin balance. There is a machine that spins the front wheels at speed. You would then add/subtract and position the weight accordingly. On the back wheels (non-posi)
you would simply prevent one rear wheel from turning, and run the car in gear to spin the tires one at a time. On posi-rear, you have to take one tire off and then the other. I still think this is a better way of balancing, but with the free standing & easy to operate balancers, this is an art that has gone by the wayside. It would allow you to determine which wheel or wheels the problem comes from.