HAVE RECENTLY INSTALLED 4 NEW TIRES AND CHANGED THE BACK SHOCK ABSORBERS , YET I FEEL THE REAR END OF THE CAR IS VIBRATING AND THE BACK SEATS SHAKING BADLY . ANY SUGGESTION / EXPLANATIONS.

If you haven't done it yet get the van back in for a tire rebalancing and nut/bolt check. Also, have the tread on the tires checked carefully. I have had numerous problems over the past 5-6 years with virtually new tires coming apart. I think many of the mfgrs are having tires constructed in China....and the results ain't pretty.