Thanks for the replies. Suspicions confirmed... There is somewhere around 250 lb of liquid (about 25 gal of weak CC solution) in each R4, but I have no idea how much the tire/rim weighed before that, maybe 150 lb or more? So I there's probably at least 400# each to muscle around. On the plus side, this little tractor now feels stable as the Rock of Gibraltar with loaded rears and 400# lb of box blade on the back.
I've never had a rear flat on a tractor (knock on wood). I ran my old Kubota 15 years without one, although I did have one on the front--which was expensive to fix because on that tractor all four tires were CC loaded, and the local Goodyear truck tire place was less than enthusiastic about doing the job, but at least they did it.
I was thinking mainly about the possibility of putting on dual remotes and Power Beyond (I want to add Top 'n Tilt and maybe a bucket thumb one of these days), which according to conventional wisdom requires removing the right rear wheel. However, after hearing what's involved I think if I do that I'll let my friendly but rather expensive JD dealer's shop do the whole thing, and consider the labor charge a bargain compared to a hospital stay.