You have torque steer and I have had this problem several times on the transport van. It is a broken bolt on the sub frame, it holds the engine cradle to the body of the van. Put the van in gear (brake on) lean out and look under as you press the gas brake on, tourque up the van against the brake and you will see the cradle member separtate from the frame where the bolt has broken. There is a recall campain for this failure and is in effect for the life of the van so when you take it to be fixed DO NOT let the dealer tell you the recall has expired if I can find the papers which I keep I will give you the recall number or call GM and check the recall web site. Twice the dealer tried to screw me for 7 hours labour and when GM told them to cover the job it only took 3 hours to fix MMmmmmm, customer screwing again. They will cut through the floor of the van and install new bolt ass'y.