Truck bounce

I drive a 2001 f250 2 wheel drive diesel long bed crew cab with 75000 miles. Recently I have noticed a bouncing in the seat when driving at the speed of 42 -44 mph. In have replaced all 4 tires and rear shocks trying to alleviate this problem to no avail. I also get the bounce when going over a slight bump. I have also tried different air pressure setting in the rear tires that also did not fix anything. If anyone has an idea of what is going on I would appreciate the feedback. Thanks

Have someone check the front suspension including the shocks. If they don’t find anything there, you might have them check to make sure that everything is tight at the back end.