As the others mentioned, a full fluid change out is a good idea when picking up a new vehicle that you have no idea of the service history on. Since GM trucks tend to eat front suspension parts, you didn't waste $ or time, doing that. Now catch up on fluid maintenance, and you should have a good truck that will last you awhile.
Almost everyone here has over 150K miles; many have passed 200K, 250K, and more, still running strong. If you maintain it mechanically, you'll generally lose the truck to rust before you lose an engine. But that's true for many vehicles, these days.
As mentioned, use only the GM Auto-Trak II fluid for your transfer case. You'll need 2 qts, and won't have much gone from the second one. Every other fluid, you can use a brand of your choosing. All of us have our own favorites.
Check your power steering lines, as they route toward the upper fender. There's a spot where the lines tend to rust out, so being on top of this is a good idea, because replacing the steering rack is a big job, if your truck is 4WD (did you notice the driveshaft going through the cast hole in the oil pan?)
None of the things below are 'urgent', but keep them in mind...
Replace your trans filter, along with the fluid. If you're mechanically inclined, start looking up all the TransGo threads here; many of us get a kit and remedy one of the weak points of the trans (which is an overall good unit, but it does have some weaknesses, a couple of which we can bulletproof ourselves). The trans can stay in the truck, but you'll have to remove the valve body, so familiarize yourself with the steps, before committing yourself to doing it.
Other things that go out with some degree of regularity are the 4WD encoder motors, some of the timing / emissions components on the 4.2L inline six (I'll let others go over these, as I have the 5.3L in my Envoy), and HVAC actuators (there are five different ones behind the dash).
The sixes have an issue with water dripping off the windshield cowl, straight onto the #4 plug, which causes issues. Again, talk to the 4.2L guys for more info on how to fix this properly.
If you have a V8, intake manifold gaskets need periodic tightening / replacement (common to LS engines). Only to 108 inch-lbs, though - you don't want to snap off those 8mm bolts!
Cooling fans are trouble with these - the clutches are thermo-viscous, and fail. Best solution is to replace with a traditional thermal magnetic clutch, but you'll need a tune to get rid of the OBD-II fan codes (and we have sources that will do this, on the cheap). That's what GM did, by the time the '08 MY rolled around - changed to the old-school thermals.
Ignition switches (they go without much warning, so it's good practice to carry a spare, as they only cost $25-30, and take about 20min to change out, as long as you have one.) GM / AC Delco only on these (you'll find we tend to be parts snobs here, and, more often than not, go with the 'genuine GM' replacement parts for things.)
That's about all the 'regular' stuff that has issues (at least that I can remember). You'll find no better place on the web concerning the care and feeding of these particular trucks, so pull up a chair and stay awhile! Many people here have had their GMT-360s & -370s (long wheelbase) for several years, some since new.
