Thanks all ...
This back pain is definitely not a momentary, fleeting things. It started acting up back in May when we started doing more climbing rides ... then eased up in June when we were in Canada doing flat rides ... and is back now that we're back in Tasmania and climbing again.
It starts with a pain about the size of a fist and then spreads across my back and is agonising ... and now, two days after the ride in question, it has eased a bit, but it's still there. Like DOMs, and who knows, maybe that's partly it.
I do change hand positions, and stretch, and get off the bicycle and stretch, and these things help for a little while ... but then it comes back.
I don't stand while climbing. I used to be able to stand while climbing, but I've lost that somewhere along the way. I haven't stood on a climb since probably the BMB in 2006. I did try to stand while riding on flat ground when we were in Canada, and couldn't get a rhythm going like I used to be able to do.
Yeah, I figure I need to do a whole lot more core work. I'll have a look for Tom Danielson's Core Advantage book.
I'm also looking into a better option than my current backpack.