So to update this situation - after being sidetracked by my fuel pump going out and having to replace it, the fuel filter, and the fp relay, I'm back to trying to figure out this vibration thing.
I ordered, and received, the new TC mount bushing - haven't installed it yet. Today went for a 120 mile round trip with the wife and both kids. Noticed that the vibration is getting worse, and now showing up when downshifting or 'coasting' in gear - basically when not under load.
I guess I'll get the TC mount bushing replacement job complete and go from there.
Hope that's all it is.