2001 CHEVROLET VAN Power Trainautomatic Transmission Problems

INTERMITTENT TRANSMISSION SLIP- INABILITY TO USE REVERSE AND/OR DRIVE. MY 2001 CHEVY VENTURE HAS BEEN IN FOR SERVICE AT MY LOCAL CHVEY DEALERSHIP NUMEROUS TIMES FOR THIS PROBLEM OVER THE PAST 3-4 MONTHS. THE VEHICLE HAS 22K MILES. GUESS WHAT THE SERVICE MANAGER TOLD ME AFTER TODAY'S TOW TO THE DEALER ?? NO PROBLEM COULD BE FOUND - THE VEHICLE -"OPERATES AS DESIGNED" AMAZINGLY ENOUGH WHEN MY WIFE WENT TO BACK OUT OF THE PARKING SPOT WE WERE IN WE COULD NOT MOVE. THE VEHICLE WAS MAKING A LOUD GRINDING NOISE AND SOUNDED AS IF SOMETHING WAS SPINNING INSIDE THE TRANSMISSION. THE VEHICLE AND ONE POINT LURCHED BACKWARD AS MY WIFE CONTINUED IN AN ATTEMPT TO BACK THE VEHICLE UP TO NO AVAIL -AND STOPPED ABRUPTLY. WE COULD NOT USE DRIVE EITHER IT WOULD ONLY MOVE AT A SNAILS PACE. UNFORTUNATELY THE VEHICLE HAS BEEN REPAIRED AND I'M STILL HAVING...