Frame rust out points?

[D13]

The drive train in my 87 is giving up, and I'm tired of fussing with the diesel. So i started thinking about what I really want to drive. The answer i came up with is a 4wd! I have a DD but what I want is something to pull my trailer and get supplies for my landscape projects. Like a truckload of peastone. And I hate getting stuck in every stupid little pothole, or not being able to go thru that snowdrift on the road.

So looking around here in Michigan, the home of all rust, I have found a number of trucks that have been parked because of fuel tank or body issues. I have a couple of good bodies (actually the 87 needs doors rockers and corners but the floor and mounts are great). So my real question is, what areas of the frame should I be looking for to confirn the chassis is good? I know my 84 chassis the lower control arms disintegrated while it sat for 3 years. My 81 the area right behind the wheels is missing (30 years of cow poo and road salt with no bath ever....). Any common issues you rust belt guys have seen?

Thanks!