Couple of issues...

Second issue is more important. When getting around 90mph hour, i wouldn't say its a vibration like an unbalanced wheel but it sounds like(to make comparasion) the sound a wheel bearing makes. But its only above 90. The worst part is if I cruise that speed to long and then come to a stop I will notice a smell and smoky haze in the cabin. Got home and got out of the car to see it smoking from the rear end but couldn't really pin point it so it really worries me.
Issue #2: Ever noticed any leaking from the differential, perhaps the pinion seal? Have you checked the differential to make sure it has the proper amount of fluid (remove fill plug on passenger side of case, check to make sure fluid level is at or just below the fill hole)?
Issue 2 i would look towards a bad rear axle shaft bearing which usually happens from low diff fluid, like RPM WS6 stated, you should check the diff fluid, or in your case i would pop the cover off and inspect the gears






