Major Problems with Bed of Truck - how to fix?

Replacing the bed isn't hard at all, I replaced the bed on my dually when I first got it since the cross bars underneath had rotted away and the bed was doing like yours with weight on it. I bought my bed from someone with a car lift so removal and install took no time, the longest time was removing the rusted bolts. The "new" bed from Texas with 0 rust is holding up great so far.