Having a hard time determing vibration cause

I am experiencing a good solid vibration at freeway speeds (more like 75-80mph) and I almost want to call it a shudder because I can feel it in the steering and under neath my seat.

The car is a '91 with only 58k on the clock.

I searched around the forums and took down these notes:

a) No vibration while car is in neutral and speeds >= 80 ---> engine/trans mounts/ harmonic dampener

b) VIBRATION while is in neutral and speeds >=80 ---> driveshaft, gears, rims

c) Rear bearings in the carrier of the differential

d) Tail shaft bushing in T-5 Trans


Now, I was taking a look at my driveshaft and noticed while the pinion flange has a faint yellow mark aligned with the purple dot on the DS, there is also some white paint on the rear u-joint.

Does it look like the driveshaft is installed correctly?

As for my differential, I do have an Auburn unit and it makes some minor noises when backing out of a parking spot for example...isn't this normal for an Auburn?

Thanks in advance!

-Eric

The driveshaft only has a few hundred miles on it, but the white and purple markings had got me wondering if the DS is indexed correctly or not?

I reclocked the DS and the drive line vibration went away, but the car is still plagued with heavy vibration from the front end @ highway speeds. I feel a lot of vibration coming through the steering column and dash. A long time ago I had the wheels balanced, but the car has not been driven much (800 miles).

Could it be a motor mount?

The front end has new struts, spring isolators, MM alloy rack bushings, MM steering shaft, but still has the original tie rods and ball joints.

Thanks,
Eric