Goodyear's table,
http://www.goodyear.com/rv/pdf/rv_inflation.pdf
includes pressures as low as 15 PSI for the tire in question (the ST225/75R15).
They recommend tuning the tire pressure to the max load
e.g., 65 PSI allows a max load of 2540, 50 PSI allows a max load of 2150. But I was surprised to see that that they happily recommend even less pressure when the max load will be less:
45 PSI allows a max load of 2020 lbs
40 PSI allows a max load of 1880 lbs (a close match for the ST215/75R14 load of 1870 lbs at 50 PSI).
.... and the table goes all the way down to 15 PSI !!!
So I concluded that I would be able to 'soften' the tire ride by simply reducing pressure when my 2619 isn't loaded heavily... but, by ordering with the heavy axle and bigger tires, I will have 'headroom' for extra weight which the standard 2619 wouldn't handle very well.
I plan to put on Solar (and not just the panels, but the additional batteries weigh A LOT). Also, our doggy kinda fills up the back of the 4Runner, so we'll need to pack the Trailer pretty aggressively. If the weather holds up, we should be getting it at the end of next week. So the special order 2619 with the 5K axle & 15" tires needed a total of only 6 weeks, including delivery to California. I don't think they needed to add a lot of extra time for this feature... but it might be a slow time of year, maybe they wouldn't get it done as quickly in the Summer.