how to stop the vibration (rattle) on my front drive shaft. on 1996 Toyota Previa

the rubber flexable couplins have been replaced.

2 answers
check your drive shaft bushing rubber usually thats the problem in my case, it gets louder when you use the A/C. If you replaced the rubber flexable couplings under the alternator and one under the A/C compressor thats good but it will be damage soon if you dont fix the vibration on you drive shaft. i hope this will help you.
sounds like you may have a bad axle have them ck'd for balance