Based off what the guy says, he obviously doesn't know much about the vehicle he's selling......
There is no vacuum 4x4 system on our trucks. A rare few had a manual shift transfer case, but 98% of them have a push button electronic transfer case. There are two common reasons why the 4x4 system doesn't work on the first gens.
1) Transfer case won't shift: This is caused by a little plastic piece inside the transfer case shift motor breaking. It's an easy fix if you're even slightly mechanically inclined.
2) Automatic Locking Hubs (Autohubs): These are crap, flat out, no other way to describe them. You'll know if an Explorer has autohubs because the front hubs have a sticker that says "4x4" on them. You can replace the hubs, but we're talking on the order of $400 - $600 to do it and they're still junk.
Of course, there are other components that can cause the 4x4 system to not work, but by far those two are the most frequent. Easy way to check is to simply push the 4x4 button on the dash. If the LED next to the button comes on and a light on the dash lights up and displays "4x4", then chances are the shift motor is OK. If it still doesn't have 4WD, then the autohubs are most likely toast. Some Explorers and early 90's Rangers came from the factory with manual locking hubs made by Warn - they're 1000x better than the autohubs. Some junkyard searching can usually net you some manual hubs fairly cheap - I got a pair off another Explorer for $30 in good condition. If you decide to go aftermarket, Warn and Mile Marker are about the only options. Mile Marker is about $100 cheaper than Warn, but the quality isn't as good...but Mile Marker is still a good product overall.
You'll also need a spindle nut conversion kit, which can be found for as little as $15 per side, so you'll need two kits (one per front wheel). Neither fix is overly complicated....maybe an hour or two on a Saturday at most.
EDIT:
OK, I actually read the ad...he says the autohubs were replaced last year. That's nice, they're still (and always will be) junk. However, chances are that it's not the hubs which leaves us with the shift motor bushing as the likely culprit. There is a very detailed write-up with pictures in this forum that gives you a step by step guide on how to fix your shift motor. You might need to spend about 25 cents on a tiny piece of vacuum hose to fix the bushing and about an hour of your time - it's really easy.
Overall, the truck looks like it's in pretty good condition and $800 is a good deal. One thing I'd like to point out is that since it's an automatic, that means it has the A4LD transmission. The A4LD is VERY temperamental and is prone to breaking down if you look at it funny. OK, well maybe not that bad, but if you don't baby the transmission then there's a good chance that it will crap out on you. Could be that there is already something wrong with it and that's why he's trying to sell it. Do a search for "A4LD" on these forums and you will see page after page after page of problems with them. I personally think that the A4LD wasn't really designed to be on these trucks...maybe a 4 cylinder Ranger, but not these.
Regardless, there's a wealth of information and resources available here to help you out if you do decide to buy it.
