The tough part is going to be the MPG and the cost.
You basically are faced with this dilemma:
* <$6k price
* 4wd
* 20 MPG
* Reliable & reasonably new (say, <15 years old.)
Now pick any Three of the above and you have lots of options.
A 1st gen 4runner with a 22r and a 5 speed will give you 25mpg on the highway. Certainly you can find them for under $6k. The problem is, you're talking about a vehicle that is at least 25 years old and will most likely have somewhere north of 250,000 miles on it. Towing is also going to be dicey with a 4 cyl.
To be honest, you're going to have to dump one of your requirements, and it will probably have to be your desire for 20mpg. An older Subaru Outback or Forester might be close but IIRC they are not rated to tow very much. The newer ones are better tow vehicles but will bust your budget. For a pre-2002 Subaru you have to consider the high possibility of a head gasket failure, its a notorious weakness of the 2.5l flat four. It had pretty much been fixed by 2002, though.
FJ 80s get horrible gas mileage -just check the threads on people trying to squeeze a few MPG out of their 80's. I think you can get 15mpg out of an 80 if you are going downhill with a tailwind but otherwise, figure on 12-14 and that's before you start modding it with bigger tires, armor, etc. There are lots of folks with 80's who will candidly admit to getting 10mpg and being happy with it.
A 3rd gen 4runner would actually fit most of your requirements including price, the difficulty will be in finding one that isn't beat to hell, especially East of the Mississippi. Furthermore, a lot of 4runners sold South of the Mason Dixon line are 2wd, so you have to watch for that. Also, while 20mpg is possible in a 3rd gen 4runner, it's not something you can count on (I got up to 23 in my 99 with an automatic, but that was under very favorable conditions, i.e. flat roads, highway speeds, and holding the cruise control at about 62mph.) 16-18 MPG is much more common in that class of vehicle.
Since you already have a Ford have you looked at a 2nd gen (1995-2001) Explorer? They can often be found very cheap, used by a suburban soccer mom, and sometimes even with low miles. For your uses, I'd stay away from the V8 model, just because I think it has full time 4wd (not sure) but I know it only has a single speed t-case. Also the V8 isn't going to be great mileage wise. If you look hard, you might even find one with a 5 speed manual tranny, which would give you a 2 speed T-case and would avoid any issues with the transmission (not that I know of Ford automatics being troublesome, just that modern automatic transmissions in general have a lot of electronic parts and other things that can fail, vs. a manaul which is pretty foolproof.)
The Chevy guys can chime in about whether the S10 Blazer or Trailblazer is a decent vehicle. They should fit most of your requirements (except, again, MPG) but I don't know enough about them to recommend one or the other.
The Montero Sport is another possibility, but I can tell you from experience of owning a Montero in NC, there isn't much in the way of aftermarket or dealer support. They were just never very popular in the Eastern US (I had a 1990 Montero when I was stationed at Fort Bragg from 1992-98 and people would actually come up to me at stoplights and say "what is this thing?") A Montero Sport is about the same size as a 3rd gen 4runner or Nissan Pathfinder.
An Isuzu Trooper will also meet all your requirements except MPG. As with the Monty, you have to content yourself to owning a vehicle that has very little US-based aftermarket, so you'll have to get "creative."
Another sort of oddball might be the Isuzu Rodeo. They are often available very cheap, but sometimes that means it's a trouble-prone vehicle. I've heard both good and bad about the Rodeo, but no personal experience.
A Tahoe or an Expedition will fit your budget and your towing requirements but not your MPG requirement (but again, IMO nothing that is <15 years old will.)
Seriously, if you are willing to give up 20mpg there are a whole bunch of possibilities.
