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You didn't say what is leaking for sure. You said oil, but may mean transmission fluid? However, first go and get some engine cleaner from the parts store. Spray everything that is coated with oil and then hose it off. Once the bottom is clean, drive the vehicle for a mile or so and look for new leaking. Once the area is clean, the leak(s) should show up well. Also, being clean, it is much more pleasant crawling around under the truck trying to find the leak.
Other than the usual places like main seals & gaskets, here are a couple of other things to look for. I had a couple of bad oil leaks in my truck and because everything was so greasy under there, I didn't find them until I pulled the motor for other reasons. My oil pan had a patch of rust on it and the rust had made a couple of pin holes through the pan. The other leak was from the vent hole in hte mechanical fuel pump. If you have access to a steam cleaner, USE IT! Otherwise, use engine cleaner and a hose as suggested above. Hot water works better if you can get it. Try hooking up to your washing machine hookup. If you have a bad leak and you have been driving it that way for a while, you'll probably never find it until you get things clean.
If it is a 302, I would bet it is the rear main seal but might try valve covers first as they are also common for leaking. 302 seems to always get a rear main leak after about 80,000 miles. Doesn't hurt anything but does make a mess. I just changed mine finally a few months ago. Pretty easy but you have to pull the tranny back a bit to get in there.
As Lee said, common problem is the valve covers and they're easy an easy thing to check first. They're not torqued down much and have a tendency to loosen up after a while..............
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