@timethief Yes! Thank you SO much! Surprisingly, it was a post with only a photo that I had used Posterous to upload to WP. I found an easy way to find the problem post: while viewing your blog, look at each post individually instead of the default of 10 or 15 posts per page (so that it is the only post on the page.) The sidebar worked on every post EXCEPT for the problem one, which the widgets looked absolutely crazy, cluing me in to the "bad post."
I have had strange formatting problems with other themes before this, and grudgingly kept going back to the one I've been using for a couple years (Redoable Lite) because it was the only one that seemed to lay out correctly. I'm betting now it wasn't the themes but a hinky post here and there (probably those I used Posterous to upload the images from.)
Thanks again, you're a lifesaver!