Feel free to mention any part of my advice that's wrong!

You're already running close to or over the rated GVW which is just one problem, could have caused or exacerbated failing rear spring components. Rear springs not able to handle their load will cause huge oversteer issues or tail wagging that is felt in the steering wheel. Get those sorted out before looking elsewhere.

With your current needs and use I'd highly recommend a rear sway bar along with making sure the front bar's bushings are in good working order too. Even a puny stock rear sway bar is a huge improvement especially on an extended body van--which is what I drive.

Also the full sized rear spare tire can/does introduce its own handling differences on these extended body vans. Nothing "fatal" but something to keep in mind.

 

Thanks to all for the suggestions. I wanted to give you all an update and maybe it'll help someone. I ended up getting a redhead gear, and boy what a difference! It was pricey, but totally worth it. There is zero steering play now. I also installed roadmaster active suspension on the rear at the same time. That too was a huge improvement. The bump steer was gone, the wandering was gone, but I still had an almost to tight feel that wouldn't return to center. It still felt like it was fighting me to constantly make small corrections, and would stay in whatever direction I had previously steered. I now know this condition is called memory steer and is a common thing to twin I beam fords. Today, I disassembled the entire brand new front end, and sure enough, the lower ball joints were very stiff, and the center link was stiff too. I worked grease in them, freed them up a bit, then loosened and re torqued every joint in the front end to moog spec. Then I set the alignment by SAI degrees, by adjusting the upper bj camber sleeves until both sides were at 8.0 degrees. 50 mile test run on the interstate tonight, and it is SO much better. For those who don't know what sai is, it's steering axis inclination. Good alignment machines read this measurement and it's too often overlooked. I let my wife drive on the way home, and she loves it! She feels safer and the kids have noticed a difference in the ride. They tell me it's softer over harsh roads. It's like having a new van. Thanks, all!

 

I also realized that tire PSI makes a giant difference.

I was at 41 psi on all 4 tires.

I went up to the max 50 psi on the rear tires and left the front ones 41 psi.

Made a huge difference.

Glad you got yours figured out. I am still inching towards better steering in mine.

 

Yes it does! mine's a 350, and I keep the tires at 60psi on front and 80 in the rear at all times. hope you can get your van straightened out!