I have a 98 E350 Winnie. I also have a 2003 E350 both with V10 for towing my race trailer. Got the used Winnie last year with 50k miles. Every time I lightly brake at 15 mph or less I get a decent shake in steering wheel. I had to change all tires and thought that would fix. Nope. Even swapped some tires. I am a machinist so I put an indicator on rotors and they seem ok. I used to machine rotors at auto parts store. I then did an adjustment on steering box because there seemed to be some play after I looked and treated all linkage. Plus I have a duplicate I can compare to. I did adjust steering box but no real difference. I do suspect the box but I still may cut rotors. When I slightly lean on brake in wobble it goes right away. On the road I had no problems just when I am in very slow mode and rolling up to stop sign.

 

Sounds like the rotors have some high spots to me.