Well in the spirit of the thread, which ones that would meet the OP's criteria?
I've put about 170K miles on two different Jeep 4.0L engines. My wife ran a '96 ZJ from about 30K miles to 140K without ANY issues other than battery, brakes, shocks, tires, etc. I don't know that you'll get more reliable than that. I think I do remember a trans coolant line or something, but don't remember the specifics other than it cost about $50 or similar. Not the best MPG, I'd say we averaged about 14-17 mpg or so. The 4.0L in a ZJ is not exactly a good match, but it'll get you by.
I bought a '91 XJ (aka "the Toolshed") at about 120K (?--can't remember very well) and sold it at about 185K (motor was DONE, but still ran---had so much blowby that it would dump a prodigious amount of oil into the air filter on a trip over an hour). Other than self-imposed trail repairs, I only had one major issue with that vehicle. A mis-diagnosed CPS cost me about $1200 in parts/labor over a few weeks.
With smallish tires, I could knock down 18-19 mpg. That figure went down as the tire size went up. With 35" tires, a SUPER tired engine and 4.88 gears, I was still getting about 14-15. I really liked it with 32 MTRs or 33" SSRs and 4.10 gears. Very capable, would tow our tent trailer without issue and wasn't a beast to drive everyday. I think I got about 15-17 mpg with either of those setups. There's just the creaky uni-body thing with the XJ. Not exactly the best design. They're a lot like building a drag racer out of an early Camaro/Nova or Mustang. The unibody has a lifespan. When it starts cracking and creaking, buy another and move all your junk over to the new platform. You can gusset/rebuild and chase cracks as often as you want. The stresses will move around the body to areas you haven't addressed yet.
I think if I started over again, I'd look for a TBI K5 and one ton axle it with some 37-38" tires. They're a little big for some trails, but I think that'd be the most reliable, easy to find parts for vehicle that there is. There's plenty of room, cheap to buy and build and somewhat capable.
You do have some reliability issues just built into the K5. Frames aren't exactly known for their strength esp. around the steering box. The 700r4 could use a good rebuild so YOU KNOW it's good to go. For the most part, the TBI motor is good to go. Decent power, but not exactly a powerhouse. You can use your imagination to figure out what would swap/work on one of those, but then you kind of end up with a "custom" type rig.
I've got at least 250-300K miles wrapped up in GM TBI motors over the years. SUPER reliable but nothing exciting performancewise. My wife and I put about 120K on an '89 Camaro. Put a cooling fan in it other than brakes, tires, shocks etc. Since it was still "nice" when we sold it, we got some decent $$ out of it.
I've been looking at some of the expo-type rigs. The fullsizes intrigue me a little. A crewcab pickup could be pretty nice, but you're definitely trail limited. You do have some pretty heavy duty parts there, though. Some of the full size builds are pretty clever but I'm not sure I'd want some in some of the trails I've seen in/around the swell with the switchbacks and dropoffs/washouts etc. They could be an asset at times, but way too much pucker factor at other times.
Other thoughts I've had:
2nd Gen Dakota with a crewcab and 5.2 or 5.9L V8 and SAS would be pretty decent sized. Fairly capable and kind of a decent "swiss army knife" type of rig. I'm not sure I'm sold on that, though.
mid-sized Blazer/Jimmy (they have HORRIBLE reliability reputations, though) and SAS could be intriguing as well. (I don't get the reliability reputation as the 4.3L V6 is a COMMONLY swapped motor/powertrain--not exactly powerful, but adequate)
I test drove an older (similar to your little pickup) Tacoma crew cab with IFS. I just am NOT comfortable with the seats that low to the floor or something in the Toyotas. (something about that cab is just not ergonomically correct for a guy my size with craptastic knees) I did drive a newer Tacoma (the wider versions) and it had decent power and was better riding for me than the older ones but just couldn't justify the price tag for my current uses.
Ultimately, I bought another tow rig for the Jeep/trailer combo and daily drive a deezel pickup (kind of annoying, honestly). I'm attempting to see how "streetable" I can make my "Jeep" (TBI 350/700/D300 and one ton axles with 40" MTRs) without sacrificing too much trail ability. (speaking of that, I need to call Carl at Tera). I'd like to get it to where I could drive it through the 'swell in a reasonable level of comfort. EZ Rhino's CJ8 is about what I'm heading for there. I've got a solid start, but a LOT of work to do. Storage of gear will be an issue there, similar to your FJ40. Cowboy's used to roam these areas with everything they needed on themselves and their horse. I think it could be done with my current off-road rig, but I've grown quite accustomed to SOME luxuries. (I hate to admit that, but it's totally true) Time will tell how it works out.