I would put cables on all 4 rear tires. I think a set is about $35 at Wal Mart for my 16" rims, don't know if they carry 19.5" rim cables.
The reason is that the tires are slightly lifted off the ground while on cables. So if you look at each tire, the ones without cables will be lifted off the ground by the tire with cables, and this would take weight off the tire without, and place that weight on the tires with cables.
Also if you measured the outside diameter of tire with cables you might get say 90" while tire without cables is only 89". So tire with cables will want to travel farther than the tire without cables, and cause problems with that as well.
Good luck!
Fred.
