Pick Up towing a Van

Borrow a Brake Buddy if you don't already have one. I see you're already removing the driveshaft. Keep something IN or Over the back of the transmission or you'll have fluid running out. You CAN duct tape the rear universal caps in place so they don't get lost and dirt stays out of the grease. Then tie the driveshaft in place but away from the differential. It's then that plug you need. Just be sure fire certain it's very well secured. And nobody tries to drive the van...

I'd add a bunch of weight in the bed of the F150, sand/concrete bags, something to increase the rear axle weight.
I pulled a Jeep CJ with a 1 ton SRW truck and could feel it back there, you have the weights reversed.
Key in the UNLOCK position so that the front tires can steer in the turns. If not it will push the rear of the truck.

I have towed my Geo Tracker (approx 2500 lbs)with a full size E150 Ford van (approx 4500 lbs) and I could definitely feel it moving around back there. On my motorhome, no problem. I would be very careful about towing a vehicle about the same weight as the tow vehicle. JMHO

When towing a vehicle, the towed should not weigh more than the tower.
What are the weights of the two vehicles?
Based on an experience my son had, I would not do what you are describing!

Front wheels must be free to turn. However. Remember that you cannot back up, as the wheels will act as casters, and try to turn