to vrsmith:

I think that the conventional wisdom would indicate typical W126 rear-end disease, causing your wandering and inability to relax at speed.

I've experienced that annoying feeling myself - and when I have a fw extra $$, I will: replace the differential mounts and the rear subframe mounts and bushings. This is supposed to do wonders for the W126.

The available literature says that 90% of all W126's with this fix ride like new again-with relaxed high-speed driving.