We've had a few GMT400, GMT800, and GMT900s, and across the board they've been outrageously reliable and outstandingly comfortable to travel with. Three Tahoes, two Yukons, and two Avalanches, I'd suppose, makes me fairly well-versed in the GM truck world.
We did have a '99 Tahoe for a little while that my Grandfather picked up for pennies (relatively). Among all the trucks that he's had (mostly Tahoes and Yukons), I'd say it was among the best-looking (emerald green with a tan leather interior), but it wasn't exactly the one that I liked riding and driving in the most. It was comfortable, sure, but the refinement just really wasn't there with the interior and the NVH levels are quite poor compared to their later revised versions.
The GMT800s that we had were quite a bit better. I spent a lot of time with the '03 Tahoe, and it was leagues ahead of the '99 in terms of quality, I thought. The Yukon of similar vintaged that another Grandparent had was just a little bit better, like most GMC things, mostly a ride difference that would be barely noticeable to most people. However, of all the GMT800s, I'd say I liked my Dad's Avalanche the best. It was by far and away the most-capable of the three, and I certainly liked the added benefits of the mid-gate. My only real gripe with the Avalanche was that the back seat was absolutely dreadful, sitting a bit too upright and zero adjustability makes for terrible trips.
As for the GMT900s we've owned, well, it's a similar jump forward in quality and performance. Fuel economy is top-notch in the Tahoes, Yukons and Avalanches we've had, regularly exceeding 20 MPG in mixed driving. The NVH is down quite a bit, ride substantially better in all models and trims, and they fixed that damn back seat in the Avalanche. I'd say it's a draw on which model is better here, it really depends on what you want to do with it. The added power to the Vortec 5300, and the extra gears in the 6L80E go a long way to make the trucks much better to drive, tow, and work with.
Apples to apples, you'd have to get a very well-optioned Expedition (ie, Eddie Bauer or higher) to be on the same level as some of the later GMT800 Tahoes and Yukons that are LT/SLT trim or higher. The performance of the Vortec V8s were also quite a bit better, especially in terms of power and fuel economy, and should be more than enough to meet your needs.
Oh, and don't worry about the transfer case stuff. Any reputable GM dealer should have it available, and it would be quite rare to have to replace all of it outright. I'd assume finding any in rear-drive, outside of municipal sales, would be quite difficult. At least 95% of them up here in Michigan were sold off dealer lots with 4x4. The on-the-fly switching between 2WD and 4WD is quite easy, and rarely has problems.
