Saab Repair: Front end vibration in 1990 Model 900, front end vibration, high speed test

Question
Am trying to figure out cause for vibration at front end of my 900 Saab.  I noticed this starting a couple weeks ago.  Vibrations begin when I reach about 50mph in 4th/5th gears, gets worse as speed increases.  Happens when power is applied, i.e., when I take foot off accelerator or coast down hill, then vibration stops.  Front of car feels like it shakes back and forth, feel it mainly through accelerator pedal.  Car has about 155k miles on it.  Had wheels balanced a couple months ago, current vibration does not seem related to wheel balance problem.  I had car in shop last week.  Mechanic only had time for a brief look at it.  He checked transmission fluid, it was black and thin, but very little or no metal debris.  Put new fluid in and vibration seemed better during test drive to 45mph.  Mechanic thinks may be drive shaft bearing related.  Driving over last couple of days at 60+/- indicates vibration still there, gets pretty bad at 65+.  Mechanic and I are going for high speed test drive today and he'll take another look at it tomorrow.  Any ideas in meantime?  Thank you.  

Answer
Yes, this happened to me too. I beleive you have impending drive shaft failure. What happens is that the driveshaft is not running "true" and is out of balance. This is potentially dangerous, when my driveshaft failed I was barely able to hold on to the steering wheel, it shook so bad....Get your driveaxles checked ASAP!