Rear End Shimmy @ 62mph?

[350SL4spd]

'75 450SL.

Drives & handles fine except btw (an indicated) 62-65mph she has a pronounced shake in the rear end, then it goes away. It is definitely not in the front suspension. I just had all four tires spin balanced & a new front flex disc.

The differential mount looks good (& there's no clunk anywhere).

Bad rear flex?
Bad carrier?
The new front flex had longer bolts than the ones I took out, but that wouldn't cause such a speed specific shudder...

I should mention that the car has 15" MB rims (from a later 560SL) & the tires are wicked wide (225's), but they're fairly new & were just balanced...

Am I missing something?

Thanks!