WOW if that really is a Dana 60 you could make some money off that. Bookmark this link...
http://www.pirate4x4.com/tech/billavista/60_front/
it should help you identify it from a Dana 44, which for $100 I'm thinking it is more than likely a D44, or a typo on the price for a D60. From that link, you want to look at #3 in this pic...
It will say "60" if it really is a D60. Go ahead and assume it is going to need kingpin bushings & bearings, those arent very expensive. Turn the pinion yoke back & forth a little and get a feel for backlash of the ring & pinion. There should be a little "clunk" noise and no movement of the axleshafts but the movement should be almost unnoticable. 1/16" movement is borderline. Other than that look at the yoke for broken u-joint cap keepers, pinion seal for leaks, broken wheel studs, etc. It is going to be hard to tell if the wheel bearings are bad, unless they are really bad. Make sure all the brake hardware is there. Make sure the caliper shim bolts arent broken. Look for gear oil leaking out the axle tubes, the internal seals are a bitch to replace. Ask to remove the cover and check the condition of the ring & pinion, count the teeth and verify it really is 3.73, and check the condition of the carrier and even the oil. If it is brown it may have been in the water.
For the rear look for leaks at the drums. The seals arent hard to fix but may also be an indicator of a bad wheel hub bearing. Same as the front check the yoke for broken u-joint cap keepers, and check for a leaky pinion seal. You may see a little at the pinion housing to 3rd member mounting surface, no big deal. Same deal as the front with checking backlash, if the pinion moves more than 1/16" the backlash may need tightened up a little which is much easier on the 14 bolt. Again ask to remove the cover and check everything out. You wont be able to tell if it has a LSD, the way the carrier is made the mechanism is concealed.
And think about the gear ratio, are you sure you want 3.73s? Changing the front & rear ratios, even doing it yourself could run over $600 for the parts & setup kits. Even if the D60 turns out to be a turd I would get it, you can make your money back and then some,
as long as it really is a D60.