School me on Trucks NS!

Hi Newschoolers, I know this probably isn't the best website to ask this question but I know there are a few members with some good truck knowledge on here who could maybe give me some advice or point me in a good direction to find good advice.

So by mid/end of summer i'd like to purchase a new vehicle. Right now i'm driving a '99 subaru legacy wagon and while it's a good car, it's nearing the end of it's life and I don't want to be stranded next winter with a blown motor or something and not be able to find a good replacement vehicle. Also my lifestyle is just more conductive to driving a truck. I live in the mountains and spend some good time driving unmaintained/maintained logging roads in most seasons for hiking, photography, ski touring, and just general mountain town fun activities.

Basically the key things I'm looking for in a truck are:

-Reliability (obviously). What are reliable vintages of motors and trannies for each company? I don't feel like buying a lemon just because it's cheap. Along with this question (I know it would be a lot easier to answer once I narrow down my search but..) What are key things to look for maintenance/problem area wise when buying a used truck?

-able to carry 4 passengers (+driver) somewhat comfortably. Extended cab or whatever. No standard cabs here.

-6 or 6 1/2 foot box.

-highway drive-ability. living in kind of a rural area, i do a lot of highway driving. I dont want to feel all janky and sketchy going 80 mph on the highway.

-goodish gas mileage (20+ mpg)

-4x4, locking diffs? can you even buy trucks with locking diffs? who makes a good 4x4 system or are they all comparable? what are the top speeds for different 4x4 systems?

-gas or diesel? i wont be towing too much but reliability and fuel economy are a plus. I've always wanted to convert to a biodiesel too when I have time in the future.

-towing as previously mentioned, i dont plan on towing much besides my parents' boat a few times a year and the odd trailer. I dont plan on buying a camper or 5th wheel or anything.

-budget max - $$ probably about $13000. I'm not sure where I'll be working this summer or what I'll be doing next fall/winter but $13000 is what I'm willing to spend give or take.

-i definitely don't want a big lift but i will probably level it and maybe lift an inch or two just to fit some slightly bigger tires on there and get some more clearance. I dont want to be building a full on rock crawler bush rig or anything.

-i'm not set on a certain size yet, just want to fit into the requirements i said above. any ideas about that are very welcome too. looking into 1500 size dodges, chevys, gmcs as well as tacomas (what im leaning towards), tundras, titans, and frontiers.

I think that's about it.. There's a lot of info there but like I said before, if anyone can provide some good info or point me in the right direction, I'll be very thankful.

Thanks NS!