Thanks for you comments!
Freestone: I definitely don't intend to carry 50 lbs (nor was I on this trip, at least I don't think so; I hardly ever know exactly how much my pack actually weighs at the beginning of a trip; too bad they removed the scale at Whitney Portal...). On the other hand, I do have a really hard time keeping the day-1 pack weight much below 40. The idea is that if a pack's suspension can keep it in the right place on my back for a few hours, with 50 lbs of sand inside, then it'll probably keep it in the right place all day while going up and down in the mountains.
Daisy: Very cool to hear that sciatica can be sensitive to pressure change! That would explain a great deal. I do sort of know what's causing it: when the sports-medicine gal looked at my MRI back after I had really bad sciatica, she said, "well, B2, there are a lot of reasons you might have trouble with your back!"
Turns out that there's all kinds of nasty bony arthritic stuff down there getting in the way. Not much I can do about that except those exercises, I guess..
Moral of story seems to be: be careful of long-term sitting immediately before backpacking. Will do!