Put the tranny mount in, no change.
Went ahead and replaced the rear u-joint, no change
Re-phased the driveshaft, now vibration is only at around 3000 rpm and 55 mph.
I did notice the slip yoke has .015-.025 of movement, measured at the u-joint ear. If I slide it all the way in, about an additional 1" from it's normal position, there is no movement. Is this normal? I can't find a spec for this in the service manual.