Thanks for the suggestions Jorge. I don't know exactly when it happened, but there was no extra weight on the rack or rear part of the seat. Of course, I weigh 260 (probably 280 in full ATGATT) so that's not insubstantial. But my weight is usually more forward.
I'll try MAP Engineering and HPN. I have the HPN tank on the bike. Good stuff, but expensive.
About 90% of my riding is off-road, on dirt tracks or cross-country. It's not like I'm doing jumps or anything, but many hours of very bad dirt roads or bumpy pasture land at 25-50 MPH, so a lot of rattling and hits.