I posted this over on the G 503 board , but it applies equally to any jeep frame or even a ladder style frame from any conventional vehicle. I reference the TM drawing several times, that is the same drawing that is in your FSM Factory Service Manual. There are also some instructions in the FSM about this process.
The question was asked how a guy could check a used frame easily before purchasing it, at a flea market or the sellers garage...Assuming you have a nearby piece of flat, level concrete or blacktop, this is what I wrote:
Four jackstands and a level piece of garage floor are all you really need, besides a good tape measure and a plumb bob. Set the jackstands at equal heights, and place them just ahead of the rear spring front hangers and just behind the front spring rear hangers, at exactly the same spot on each side. If the frame won't sit on all four stands, you have a twist problem already. For a quick check, once it is on the stands, (actually you could do this on a level floor with the frame sitting directly on the floor...) check the height of the front bumper ends and rear crossmember.. Each end of the bumper , if mounted, should be the same height from the floor. If you have access to the TM drawing and some patience you can actually check the heights of the spring mounting areas, but if you are just doing a fast check to see if a frame is OK you can usually assume if a frame sits level and is in square it hasn't sagged much. I'd still eventually check the heights before installing a tub on an unknown frame.
Next we need to check for square..
What you want to do is use the plumb bob to drop points to the floor from each body mounting bolt and matching suspension mounts. Be careful to use the exact same part of each hole for the string. Mark an X on the floor where the bob hangs just above it. Transfer each body mount you see in the TM drawings to a point on the floor.
Once you have dropped measurements, you can move the frame out of your way and measure away. First, check the lengths between matching points on each side. For instance the distance between the point you dropped for the front spring hanger and the rearmost spring hanger on the right frame rail should be "X" and the same dimension "X" for the matching two points on the other side of the frame. IF one side is shorter, the frame may have been smashed shorter on one side. You can keep measuring lengths for shorter areas, to see just where the damage is concentrated.
Next, check the diagonals. THey should be equal. These would be the diagonal lines you see in the TM drawing. IF only one area of the frame has unequal diagonals, you may have sidesway in that section. Sighting down the side of the frame may reveal that.
If as you go down the frame checking diagonals, you find a pattern of them being off the whole way down the frame, you might just have a frame in the shape of a diamond instead of a rectangle. ( If that is the ONLY problem you see it is fairly easy to push back with a portopower or two. What happens is someone tries to push the jeep on one corner or it hits something on one corner and that frame rail moves back in relationship to the other. You place the portopowers on the short diagonals and push until the diagonals end up equal. )
There are many other ways and tools to use for checking a frame. One of the tools a body shop has access to is like a long ruler with adjustable pointers, to measure around obstructions. Diagonals can be easily checked with a setup like that. Also there are gages that hang off the frame with pointers at the centerline, to see if the frame has swayed sideways...Those would be a little difficult to make but not impossible if you had a need for them. By and large a tape measure will tell you most of what you need, just a little slower than dedicated frame gages.
Watch for rust and rotted sections, cracking around spring hangers, and poor repairs while you are checking that frame.
