Broad questions like this will yield a million answers. Here's mine:
Ford products, I would stick with 2V Tritons, 2000-2003+(?). Older vehicles = simpler the better, and the 4.6/5.4L were stupid simple in those years other being an OHC V8, and parts are dime a dozen. Transmissions weren't great, but reliable enough if maintained. 2004+ opens up a can of worms with potential plug issues, cam phasers, etc.
GM products, I would personally feel comfortable with any model in those years.
I don't have a lot of experience with Dodge/RAM trucks of that era, but have heard the quality is hit or miss. Again, I'd prefer a simple 5.2/5.9L over the 4.7L OHC motor, which had some issues with valve seats, exhaust manifolds cracking. Hemi was pretty solid in those early years, I've known more than a couple people that took theirs to 200K and beyond with no trouble.
On a half ton truck, I wouldn't worry about axles. I don't know of any in those years that were particularly troublesome. Some are better than others, but unless this is primarily a tow rig (or there's evidence it was used as such), it'd be low on my list of criteria for that age of truck.