Your drive plate maybe have broken its springs - this transmission uses a thick two plate sandwich to drive the it, its found in place of the torque convertor, almost every failure is related to the addition of CNG and stupid advancers on the map and TP sensors.
Find yourself some place who repair automatic transmissions - they will be able to pin point it, its a very common fault.
Just out of concern - is your car shuddering on acceleration from standstill - if yes then you have deeper problems in the transmission. Sometimes recalibrating the first clutch from the ECU works - but I am yet to find some place workshop other than myself who know how to do it.