The SADS is balanced as an assembly, so anytime it's taken apart it should be marked and reassembled the same as before. Another thing that's important is not allowing the rubber insulators to rotate during reassembly (while tightening the bolts). These things are mentioned in the service bulletin I'm attaching to this post. Another thing that can mess one up is allowing it to hang unsupported (particularly the engine side). When an end is disconnected the shaft needs to be supported. This can be accomplished by using a wire coat hanger or something similar to keep it straight. Whatever the method, the object is to prevent side stress on the flex couplers.
When the Previa was still in production & for about 5 - 10 years after, most mechanics at most Toyota dealerships were trained on this and knew how to handle SADS. Now that the Previas are aging very few make it back to the dealer for service. As a result very few techs have experience with them (and that's the dealer). When it comes to independent shops, it's not very likely that many of these techs EVER learned about proper handling (much less likely now that it's been 17 years since the last Previa rolled off the line).
I'm probably going to catch some crap for saying this, but after years of experience in this field these are my thoughts. There are some techs who are very conscientious and very well trained. Unfortunately this is not the norm. When you consider the pay grade & working conditions for auto mechanics, it makes you wonder why any intelligent person with an aptitude for this sort of thing would choose wrenching as a profession. The sad fact is most "quality" people choose something else. This leaves drop-outs, druggies, and other "less than optimal" choices for shops to hire. I'm not saying these guys are all this way, I'm just saying it's a challenge for shops to fill their tech positions with quality employees. Some of these "techs" don't care, some do the best with the skills they possess, and some are just damn crooks. The best techs tend to gravitate to the jobs that pay the most (i.e. shops that charge the most). The not so good end up working for the independents & cut rate shops. This is the reason most people buy new or newer cars after a major component fails. Most of these shops can "fumble" their way through repairs on standard vehicles, but when you throw them a "curve ball" (something unique or unusual) they usually don't do so well.
Tim