Wow great detective work. Interesting to hear and read about this. Maybe because this issue would just be popping up in US imported crusiers because of the 25 year rule.
I was able to contact them directly and they custom fabricated my passenger seat bracket. Kind of annoying because I had to pay return shipping but I understand that they advertise these as "should fit".
I think there is something different about the 75 troopy brackets vs the 75 truck brackets.
My uninformed thoughts are it's probably the drive shaft. I'd take off the front or rear to see if you can isolate as you mentioned. Maybe the u joints?
This sounds like something that happens frequently with the 100 series - I used to have it in my LX470.
Culprit is typically the drive shaft, but have also seen it caused by the slop between the hubs and the CV. Are yours manual locking hubs or similar to what is on the 100 series?