You have yourself there a gem of a truck, very nice score there. They are dependable and tough and good looking and surprisingly capable in stock form. The TBI 350 isn't a powerhouse (210 hp and 300 lb/ft torque) but they'll go, and go, and go... You also have the 4l80e automatic, which is a superb transmission, it was bolted to everything from your 350 to big blocks and the diesel in everything ranging up to school buses and medium duty commercial trucks. Good stuff.
The front axle is a 9.25 (ring gear diameter) ifs, which is fairly stout, but it's hampered by the aluminum case and chintzy center axle disconnect. The case you can do nothing about, but I recommend highly the 4x4posi lock, especially if you ever want to wheel in the snow or ford a river. GM used a thermal actuator that needs to get hot and expands and pushes a rod to engage the front axle, and if it gets cold (ie. a water crossing or cold weather) it just won't work. If you want to keep the dash uncluttered, you can upgrade to the mechanical actuator used in later gmt400 trucks.
Your transfer case is an NP241 with a 2.72 low range and is a very good case. Pull it apart someday and throw a rebuild kit (new chain, seals, bearings etc) and a case saver and you'll never need another. A case saver is needed because as the bearings inside begin to wear, they let the gears rub against the inside of the case and will eventually break through. It's not an expensive upgrade but worth it. If/when you do an axle swap, buy a slip yoke eliminator kit and you're good to go.
Your rear axle is one of two: a 14 bolt or a 14 bolt. One is smaller than the other. The smaller 14 bolt (number of bolts holding the cover on) is a 9.5 ring and pinion and will survive forever in stock form with an unmodified motor. It is a semi-floating design, meaning the axle shaft transfers power AND holds up the truck, as the bearings ride on the shaft. Its weakness is the c-clip design holding that shaft in. If that clip were to break or come loose you're in for a wild ride! (This is a very rare occurence though, so don't panic) The bigger 14 bolt is the Holy Grail of rear axles. It is pure beef. Bullet proof. Unbreakable. The last axle you'll ever need. It has a massive 10.5" ring gear and is a free floating design. That means the bearings have their own race and the axle shaft only has to transfer power, so that if you ever break a shaft, you'll be able to drive home no problem. This is the axle rock crawlers and mudders and guys that tow all want. If your glove box has "G80" as an option code, you have the gov-lok option, which is a good thing. The gov-lok often gets a bad rap for being either worthless or breaks easily, but that's the 10 bolt gov-lok. The 14 bolt (either axle) gov-lok is a different design and is pretty stout and reliable and is a real locker, not a limited slip like the 10 bolt unit. The way you can indentify which axle you have without pulling the diff cover is pulling a hubcap off. If there are bolts under the cap, you have the full float, if not, you have a semi.
Power upgrades for the 350: find some vortec heads, they flow better, allowing you to make more power across the band. You'll need a new intake to do that swap, but it would be worth the investment. Basically think of the motor as a big air pump, and the more efficiently you can move air, the more power you'll make. Just stay away from cheap stuff, like a $20 cold air intake kit. Just no. And when the time comes to do a rebuild or swap, GMPerformance has a 290hp crate motor that is drop in and only a few hundred more than a basic small block. Totally worth it. On my Yukon, when the time comes, I'm going to go with an HT383E motor, as 330 hp and 430 pounds of torque are hard to beat. It's pricey that's what you have to pay for awesome.
As for your seats, anything from an 88-98 truck or 93-99 suv will bolt right in with minimal modifications. That means you can put in electric leather buckets for relatively little dime. But a split leather bench with manual adjust would be the best way to go.
Upgrade your rear axle with a Detroit locker (if you don't have the gov-lok) and a set of disc brakes. Unless you have the semi-float axle, then find a full-float and put a Detroit and discs on it before you bolt it in.
That's the nice thing about GM trucks, parts interchangability is crazy! All the motors use the same frame mounts for the engine, all the bellhousings are the same, the transmission outputs are all the same, the rear axles all use the same spring pad/shock mounts so any axle from any truck will go in easily. And from the b-pillar forward, they're all the same sheet metal and frame.
I am actually a little envious of you and your truck, I almost bought one very much like your's years ago but my brother offered me the Yukon at a great price and I knew it's history. Happy motoring with your old Chevy! ahem, GMC...
