Determing frame straightness

[SUX2BU99]

I'm trying to determine if my frame is straight or if I have saggy springs. This stems from my box being lopsided and droops down on the passenger side. I figured I'd just throw some washers between the bed and frame and jack that corner up but that doesn't really solve the problem. But what also bugs me is that every time I jack my truck up and put it on stands, both sides of the frame do not land evenly at the same time on the stands. They are off, like over an inch at least. I usually jack the rear end up by the differential with a big floor jack and put the stands on each end of the large towing hitch receiver that's welded to the rear frame of my truck. When I jack the front up, I do that by jacking under the front crossmember under the motor and put the stands at the front of the frame rails near the bumper.

I understand that jacking the truck perfectly in the center is key to trying to determine if the truck frame is twisted or not but I'd like some kind of definitive way to tell. I also think that the frame has enough natural sag and twist in it that putting it on stands and measuring to my flat concrete floor will not tell me anything.

When I tried getting the jack eyeballed as close to center as possible under the front crossmember, I got about 1/2" difference when settling on the stands.

When I used to put the stands under the frame near the front cab supports, before the frame kicks up at the engine compartment, the amount the nose of the truck would keep settling after touching the stands in their further back position was somewhat disturbing!

The truck drives pretty straight and doesn't 'feel' like it has a warped or twisted frame.

If I try to jack up the rear right in the center, I am still biased by the condition of the leaf springs. None of them appeared to be busted when I did my recent diff re/re.

Since there is no bushings to affect how the bed sits, something else is causing the rear to droop. I want to eliminate the frame as being an issue since replacing springs is obviously way cheaper and easier than a frame!

[Lt.Del]

I don't think you will be able to tell by jacking it up and seeing which side lands first...there are weight differences, gas tank, trans weight, transfer case if 4x4, steering box weight, steering column, etc...
It may be the rubber bushings at the bed bolts.  If those bushings are compacted or dryrotted, that may show a difference in how it looks.  
Does the bed look out of alignment with the cab?  Do the design mouldings and indentations line up (from front fender, door, and bed)?  If it was spring related, the cab and bed will still look aligned to themselves, just a corner of the truck will sit lower.   If it was the bed bolts/bushings, or frame bent, I would think it would be apparent by misalignments of the bed to the cab.  There may even be more of a gap between the cab and bed on one side or top/bottom. 

[SUX2BU99]

Good questions. I thought there was no bushings at all between bed and frame. I believe the body lines between bed and cab are good but I'll check that out and report back. I know that when measuring from front corner of bed to corner of rear window, the passenger side was down 1/4" last time I checked.