Rearend suspension question 2003 Tacoma DC 4x4 V6

I have an 03 Tacoma Doublecab 4x4 V6 with a 2" Old Man Emu (OME) suspension lift kit that included the OME replacement coils springs front and leaf spring pack rear. I replaced the OME provided shocks with Bilstein 5100's. I also have a 1" body lift, Budbuilt rear track bar that attaches to front side of differential and up to the carrier bearing crossmember, Timbren bumpstops on top side of leaf springs replacing the stock bumpstops; same thing at the front bumpstops.

I have no issue with the front end, but the rear end drives me crazy on washboard roads. No matter if I go slow or fast, the back end fishtails all over the place, especially on corners. I have tried airing down the tires and it does help some + added weight to the bed, but didn;t notice much difference either. I was wondering if I had softer rear leaf springs and shocks if that would make a difference? (assuming the springs and shocks maybe too stiff??). What I have on the truck works great on the highway + I tow a boat or trailer with it often; don't want to get too soft. Just wondering what you can do to reduce the fishtailing and track better on hardpack washboard type roads? Thanks for any advice!