Here was the stupid resolution to the bad SWR readings described above.

I moved the antenna from the middle of the pipe to the driver's side. Boom, the SWR's went good.

I did NOT isolate the mount from the conduit. I did NOT isolate the conduit from the roof rack. Neither of those things seemed to do anything helpful on a consistent basis.

The SWR's only cleaned up when I moved the antenna over. I believe this has something to do with the relationship to the ground plane.

After I fixed the SWR's I went ahead and grounded the mount and grounded the far end of the conduit - it didn't hurt anything and it seemed like a good idea.

Weird, eh?

Hed