5 speed - whines in 1st-4th

[pentoman]

My '90 w201 has a whine in first, second third and fourth gears, which instantly vanishes in fifth. What's the beef?
It's the getrag five speed gearbox as fitted to the 16v, where fifth is a 1:1 ratio. This makes me assume the problem is in the transfer gears that lower the ratio in 1st - 4th since the whine vanishes in fifth? Sorry I don't know much about gearboxes. The gearbox already had a rebuild, first thing I did when I bought it, but the whine remains .

Could the problem be related to diff installation, prop shaft problems, etc? I ask because the car's diff was replaced with a used one by the previous owner, this was done at a garage that I do not have faith in. Could they have installed diff wrong to cause this problem? I heard it's possible to install the w124 diff with a shim being used as a washer - how do I check for this?

Also the car vibrates at 80+ mph, the vibration comes and goes, in-out-in-out forever until you lower your speed.

Thanks Russell