I have a 91 Previa and replaced the SAD with help from the folks at www.previaparts.com
where they have both bushings and the assembled shaft. I tried the bushings first but made a mistake reassembling it. I called and talked to them and they said just send it in and they would check it out. They sent it back ready to install. These folks were GREAT!
The installation can be done in about 6 hours but you will need an extra pair of hands. It will take more if you are alone and you're in good physical shape. I recommend the help because getting the shaft through the front mounting assembly is doing it blind and can be frustrating. Also I recommend purchasing the 3 mount replacements. They're cheap and easy to install, plus extend the warrentee on the bushings and/or shaft.
To be sure the shaft bushings are in fact the problem and to check your work after installation, run the car with just the shaft mounted. It should run smooth and true. Anything else means you have issues.
If you install the bushings only, do it on a bench and take a picture beforehand. Mine was a simple mistake but it cost me a lot of time and effort.
All in all the dealership was worthless. They denied having the bushings and would not install them even if I gave them saying they didn't work. They wanted $800 for a new shaft plus 6 hours minimum labor.
Even if you have to continue to drive it until good weather, it will not completely fail, just vibrate. No special tools other than metric sockets. I felt good about having done my own, and bought myself a steak dinner with the difference.