Got new rear tires and now my truck sways on the highway

i have a 2400lb Bigfoot on the bed of a dodge 3500 and weigh in at 12000 lbs total.

I had 4 bfg all terrain at tires load range E, sway bar and air bags and it was very stable.

I blew out a rear tire and replaced both rear tires with the new bfg ko2.

Now my truck sways a lot on the highway. Almost felt like before I had a rear sway bar. Rear tires are aired up to the max 85lbs.

Front tires are not the ko2 and have 25000 miles on them and 65lbs pressure.

What do I need to do to have a stable highway ride again?

Thanks!