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check under the vehicle for rot related problems.
frame
shocks & springs & their hardware
brake lines
oil pan
fuel tanks. Stick your head in the rear wheelwell. If you can smell fuel it probably has a hole on top or in the seam. If won't leak if the tank is only filled to below the hole(s)
Check for an oil leak, even a small one could be an lengthy repair in a 4x4. The oil pans tend to rust and the main seals like to leak.
I would normally suggest you pull the engine codes but an 87 doesn't have one normally. You can still pull codes though. Visit www.fordfuelinjection.com to learn how. All you need is a paperclip and a testlight (or analog meter).
It's pretty much a gaurantee that ball joints both lower and upper go every 70-90k miles. Check them and if they have been replaced see if they have grease fittings. The stock ones are sealed and the typical replacements have zircs. A way to tell teh shape of them besides looking is to see how much the wheels go back to center say after you turn a corner etc. If it is real sloppy and you have to bring it to center yourself you probably need new ball joints. Hope this helps and good luck.
Another place you will want to check is at the side of the cab, inside, by the sides of the seats. There are plastic foot guards (for the lack of a better word) on each side of the cab. Under these is a small ditch were the wire run from the front of the cab to the rear on each side. I would take the time to pull these off and have a look. When I bought my 93, I noticed after bying it, that these were filled with sand and gunk and rusty under the wires. I sanded, rusted painted and rustchecked both of them and the front floor panels and will check them yearly from now on. I suggest you do this once a year when you own your truck.