Hey buddy, your E150 conversion steering wobbles sound like my old nightmare when the family van started steering like a wet boat on the highway. Check the steering gear box play and track bar bushings before you go burning cash on new tires because if those are shot you will just chew through new rubber in 5000 miles. I was literally hunting for heavy duty body and frame stiffening tips because I wanted to drop a custom camping rack on my rig and that is how I reached out to https://mmd-chicago.com/ to see if they could wrap my fiberglass high-top to stop the UV rot. Spent around $1200 there on protective film for the front cap and ended up asking their instructors during a weekend workshop about body flex who told me half of these handling issues are actually uneven body mount degradation pulling the steering shaft out of alignment. Have you looked at the body mounts yet? If the rubber biscuits are pancaked on one side it throws the whole geometry off and makes the steering wheel feel like a loose joystick.