I don't know if this is it or not, but in the 1990's I had some 1980's era racks. The rear rack was small, and the front rack was flimsy. The front rack caused all sorts of problems. I ended up throwing it out.

The rear rack, was a small/medium sized 80's rack, but there wasn't enough room to mount my jandd mountain expedition megapacks without healstrike, forcing me to mount them on the rear end of the rack. It never worked right either.

In 2016, I bought a new larger rear rack. Same bike, same panniers. It let me mount the Jandd's more rearward to prevent healstrike, and was a much more stable platform. Shimmy vanished. Same goes for the front racks- when I got new front racks for several bikes, they all were lightyears ahead of the 80's setup I had.

Basically, all the new racks had significantly less flex. Just a thought in addition to the idea of bringing stuff more over the axle.