bed sagging on 95 tacoma

my truck was an old construction truck and its beat to hell, specifically the bed. im gonna try to fix it this summer but i dont know whats wrong and i cant find anything online as to what the problem might be. looking for ideas as to what the problem is and how to fix it. thanks

 

edit: the back of the bed sags, i can grab it and move it up and down with very little effort. could be why the drivers side body line is fucked up, but im not sure

 

Get under it and look? Bed rotted out at the mounts, frame rotted out and cracked, missing bed bolts, frame bent?

 

I second the vote on the frame rotted.

 

Some pics from underneath that back area would most certainly help

 

Yup, pretty simple, just have to put in the work....
get on your back with a flashlight while your girlfriend lifts on the bed of the truck,
if its as bad as you say it is, it will be obvious if you take the blinders off.
Internet isnt going to diagnose it for you.....