Scooter's right. I ran 10-12 psi on my 72 BBC Pickup. More air causes uneven wear (hey, that rhymes) and sometimes bouncing. I thought maybe you were running less than that. Try dropping the pressure a bit.

Mickey's sportsman tires are not radials. They have a radial version out now, though.

Bigdude also has some good advice. Those MT fatties are NO good in the wet. I hit some heavy water in my '72 PU and went across three lanes of interstate and and both shoulders before I got it straightened out. Luckily, there was zero traffic.

Try the lower pressure, and make sure you don't have a trailing arm or something else loose in the rear suspension.