At VERY low speed thats got to be a drop arm or track rod issue.
If there at higher speeds then its got to be something to do with balance and probably the wheels.
 
Yes get it up on a lift and check thoroughly. I had a serious weave on a Merc, turned out to be a siezed bottom ball joint, and on Merc they are known to flail and stick the lower wishbone into the road surface for an instant u-turn. Weave was caused by the PAS fighting the siezed joint.
 

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