I hope you'll be able to run all 4 at the correct pressure now, and save by less soil damage.
Sounds like we need to be more careful specifying wheel equipment for tractors we buy. I buy 2nd had, but the last two have both come with no back wheels (previous buyers wanted to keep those silly 65 series). The 64 wheels came off the previous owner's new tractor. I had specified the tyres, but never gave the wheels a thought.
My 68 wheels are welded centre 18.4R42 which we use with duals, and have never had a problem. Previously these same wheels were on a 2wd 4040 - with 5.5 tonne on the back axle before hitching anything. Still no problem.