Well I haven't been able to get the car in the shop yet to inspect the axles, but that will come.

In the mean time I went ahead and purchased a spare low mileage diff thats in perfect condition.

'fingers crossed'

 

Update;

New axles (DSS stage 2), no change

New rear diff (oem pumkin), no change


Anything else I should look at?
I was thinking alignment, strut(s), control arm bushings.

 

Check "upstream"and "downsteam" from diff & axles.

The propshaft is balanced.
Maybe the little plate they weld on to balance it with is missing?
Maybe get it re-balanced?

Check that the propshaft is mounted correctly at diff & transmission.

Transmission mounting points.

Oil filled diff mounting dampers.

Check upper & lower control arm bushings.

Check upper & lower ball joints for play.

Check shock absorber assembly.
Make sure the spring is mounted correctly.
Make sure the shock absorber assembly is mounted correctly.

Allignment.
Being lowered might make it more difficult to get within specs and / or equal values.

Buy a new S2000

 

LOL!

Thanks for the list Spitfire; I have all weekend so I will go over as much as possible and also try for an alignment.

 

Ever figure it out? I have the EXACT same problem, vibration around 35-40 mph and it goes away then comes back at 70+ mph.