The problem is in the forward lower control arm bushing. It needs to be replaced because with your tire package a brand new bushing will likely not be stiff enough.
The extra loading caused by your tire combination probably would be best dealt with through an aftermarket high performance bushing (if such are made). I have dealt with this issue on a 140 chassis. I had a fellow here in Florida make me some hard bushings for a car with 20in wheels. Did the job with the vibration but the design loosened after a year and we haven't decided what to do next.
Bosch Master, ASE Master, L1
33 years MB technician