I built a rack that holds 6 Ford 9" Rearends. Its all done now, with all 6 axles hanging on their backing plates. It rolls around on Harbor Freight castors, but not very well. 4 wheels, rated at 330Lb each... Am I overloading them?
4 x 330 = 1320lb total capacity If,for the sake of round numbers,we assume the frame weighs 120 lbs that leaves 1200lbs capacity to hold 6 rear ends. So if the 9" rears weigh less than 200 lbs each,your under the rated capacity. Of course the 330 lb rating is probably optimistic.
McMaster-Carr has a wide selection of casters and some are even cheap. www.mcmaster.com PS- Tall and skinny caster roll with the least effort. Also, some casters are made for indoor use, some for outdoor. Make sure you get the right one, it makes a big difference in how easily they roll.