Up for sale is a good used Saab inner driver.  Unit is clean and the tripod surface is spotless.  This unit will run smoothly.  This is the component that the drive axle Tripod bearing engages into. These frequently get worn internally, causing small divots right where the tripods run at "normal load, normal cruise speed".  This causes the tripods to run in and out of these as the drive shaft turns, which in turn causes a drive shaft imbalance and shake.  Typically you will feel this shake at a certain RPM or load range, maybe 55-70 MPH, at light throttle.  The vibration may also come and go as you drive.  Initially many people suspect this is a tire imbalance, but it's often these inner drivers with higher mileage Saabs!  to test?  Drive when you feel the shake, then press in the clutch, or pop the car in neutral.  If the shake goes away?  It's for sure a bad inner driver.

On classic 900's, if you order drive axles, you won't get these with them because they are bolted into the transmission.  Replacing them is a bit of a task, but is doable.  Generally, when an inner driver is bad on the of these cars, it's the passenger side.

You can easily check these! Once the drive axle is removed from the driver, run your finger inside, along the machined smooth surface where the tripods ride.  If there is any sort of a noticeable dimple, this is where your vibration is coming from!