As the title says, I'm trying to track down a source of driveline vibration in my '86 SDL.

Vibration picks up ~57MPH and is worst ~60MPH. Largely goes away above 65MPH and turns into an audible "grumble" but can no longer be felt.

It isn't a tire issue, tires are brand new and I've had the balance checked.

Not worn shocks, they're brand new Bilstein HD's. Problem was present before and after the shocks were replaced, no change.

Latest service was flex discs and center bearing. Flex discs were cracked and ROCK HARD, bearing carrier had decomposed, bearing was rough. Replaced everything, made sure the DS was aligned. No change, but the really rough gear engagement is now gone.

Checked the U-joint while I had the DS out. Was nice and tight, didn't have any noticeable ratcheting or notchiness to it.

Car is smooth as silk up until that vibration kicks in. It comes on very suddenly when you get to the right speed.

Where should I be looking? I'd suspect if the DS had an issue (bent?) that I'd have vibration throughout the driving range. Diff issues? Halfshafts? Something else?

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]