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If you thought I was about to talk about women, sorry to disappoint you. I am talking about trucks though. I was browsing through craigslist, looking for a 76/77 4x4 when I found this one in Rhinelander. In the 2nd picture it seems to me that the frame is bent into a V on the passenger side. The V I'm talking about is from the body line on the side of the bed connecting to the body line going up to the grill, when it dips down in the middle. I have seen this a lot on pickups, is it a sign of a bent frame from overloading the bed or launching the truck off of a hill or something?
usually this is a sign of squished (for lack of a better word) body mounts. if you were to take the bed off, you would probably find them to be cracked and bulgy. i have the same problem on my 76, only, just one of them is broke down like that, so the cab sits about an inch lower on the drivers side than the passengers. all it is is a rubber bushing, it doesnt effect the attachment of cab other than appearance as far as i know. from the looks of it, you found yourself a pretty decent base to work with. i would go for it.
if your feelin particularly stout, you and one other can take the bed off, i might advise bringin two or three friends to do it though. and you can get the cab up enough to change them out with a jack.
Look at the front cab mounts if you go to check this truck out. From the picture (and location) it looks like to me that they are most likely rusted out.
The front cab mounts are going to be right behind where the front fender ends at the front of the cab (hence the name). These are a common rust area on these and alot of other trucks. From craigslist it appears this is a wisconsin truck so my instinct would say that the mount there is rusted out. When the mount rusts through the cab mount bushing will actually push up into the cab mount itself and create the sagging that you are seeing. Below is a link to a thread i created while replacing my floors and cabmounts among other things.
It may be just the bushings themselves like the other guys mentioned, but since its a wisconsin truck i would really investigate that further when i went to look at the vehicle.