My guess is it is your tires, not placos tires, your tires. but, what do I know.
You have either a tire problem, an alignment problem, or deficient strength in critical areas of your suspension.
The fact that you have the size tires that you have, causes the chances of you having tire related problems to be increased by quantum jumps over normal concepts of tire responsibility. I'll say it differently. If you were driving a 300E with 195/65R15 tires you could drive with crappy tires and not notice. With your tire size you need to be perfect or you are screwed.
Because of your tire size suspension deficiencies are highlighted. Based upon your alignment specs, you either have excess deflection on one side while driving, a tire irregularity, or you have a defect in one of the other alignment values I mentioned (wheel base differences or steering axis inclination).
Either that or its something else!
Bosch Master, ASE Master, L1
33 years MB technician