My experience is with an old '79 CJ-7. Yep, 258-6 and leaf springs. I put a new glass body on it, all new bushings stock Monroe shocks, steering stabilizer, one piece rear axles, the whole nine yards. I ran 30"-9.50's on it and the best thing I did for ride quality was buy aluminum wheels. Lowering unsprung weight helped the ride. Helped....I also ran Chevy PU bucket seats and the damned thing still about beat me to death on the run to the UP and back.
Washboard roads are a challenge even in long wheelbase full sized pu's with weight in them. Slowing down is the best answer.