Well, you have replaced most of the flex items in the drivetrain, except the diff mount bushings. Guess what???. Do you still have the OEM bar in the rear. This sounds like torque steering the rear end. Something is loose back there. The W124 can be shifty when applying full power to a less than tight rear drive. Also, they are sensitive to the tire pressure in the rears. Make sure that's on spec.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.