To the dismay of all proper knitters, I switched out the needle size in the middle of the back. Instead of fighting with the size 8 circulars, at 18 stitches to 4 inches, which worked fine on the smaller width of sleeves but were a drag on the greater width of the back, I changed to a size 9 needle, which gives me 16 stitches to 4 inches.
No, I did not swatch. I admit it. Go ahead, stone me with yarn skeins. Do sleeves count as swatching? I did dig a few squares of Malabrigo stockinette out of ye old swatch box, which is a shoe box jammed with knitted squares and, if the gods are with me, the details of their needle size and gauge on an index card. But I think that I'm either going to have to commit to Huge Swatch Squares in the future, or experiment with making the back of a sweater on a larger needle than the rest. Does this happen to you: the wider your knitting, the tighter that you knit?
Here's the inside of the back. If you look closely, you can see a difference in size between the purl stitches at the bottom and the top.
My lazy knitter solution was to decrease a few stitches so that the width stayed about the same. I'm hoping that the shoulders are wide enough: they looked mingy after I bound off, but there's only going forward with this sweater. It's cold in Illinois: I need wool!