I would think the simplest way to get the 6.0L and avoid the AWD would be to get 2500 series with 4WD. The only drawback might be the stiffer suspension as I think the 3/4 tons have leaf springs in the rear. Denalis (at least my '06) have 3:73 gears and I would think you could get those in a 3/4 ton also.
Regarding the AWD front diff: My '06 Denali has around 180,000 miles on it. I can see that the front diff has been replaced on it before as it is painted silver and the dates codes on the case are from around '03. It whines a bit now. I did a fluid change when I replaced an axle seal and it was pretty dirty with a bit of metal shaving on the plug. I picked up a used "back up" diff from an '06 with 90,000 miles. The fluid in that was also a bit dirty with some fine shavings on the plug but not too bad I doubt it was ever changed. From what I have researched, these can start having issues around 100,000 to 125,000 if you do not do regular fluid changes. I have heard of these going well over 200,000 if the fluid gets changed. Some have claimed over 300,000. Rebuilding them is not too tough or excessively expensive. They are also relatively easy to get out and install. I also believe there are a couple of upgrades that cam be done during a rebuild.