I have an 84 F250 with a sagging cab drivers side rear, I have not yet had the chance to look and see why.The rockers are rusted and will definatly be replaced I think the rear floor cab floor is also rusted, are these parts available ?
Is there a mount on the frame for this ? I donīt think the mounting bushings could make it sag. Its not a whole lot of sag but compared to the bed itīs off 3 to 4 inches .

 

I just replaced a cab mount and bushing on my Ranger. Yes a bushing can make a cab sag, so can a rusted through, cab mount, or cab floor. I found someone on Ebay who had prefabbed cab mounts (seems to be a known problem), and body parts are available, it just depends on how extensive the damage, before you might have to fab some up!
If your F250 is how I think it is, that bracket and bushing should be at the rear of your cab.