I had them replaced at an autobody shop, as I wasn't fully equipped to do it myself at the time. In my case, it was worth it to have it done. It's not a hard DIY job, and probably the most challenging part is getting the spring replaced. But with the right (simple) spring compressor, not bad at all. The bushings are a press fit installation; drive the old ones out and seat the new ones. It's critical to make sure the door is properly supported and braced if you plan to leave it on. Some folks just remove the door to do the replacements, and then rehang it by lining up the hinge leaves & inserting the pins, secured with the retainer clips. If removing the door, it's best to have a 2nd person to help, as the door can be heavy and a bit unwieldly for one person, hence less chance of damage.
The full set of bushings (uppers & lowers), retainer clips, and pins for the driver's door and labor was about $105. It was nearly an even split for parts and labor. The parts were sourced from the Chevy dealer by the body shop, so there was undoubtedly some markup along the way. I'm sure the parts can be had for less if you plan on doing it yourself. I had seen the Dorman junk pins, and they were an immediate no for me. Their price alone ought to be enough to raise some warning flags.
Edit: If the vertical seal of the 3rd door is 'petrified' and in need of replacement, it's a good time to do it. But I'm not convinced how that could be the source of the leak you indicated in the top horizontal seal area (but, hey, I've been wrong before!). Have you tried the water test suggested in post #2 to help track down the point(s) of the leak? One would think that the seal and/or body area at the the leak point(s) would be visibly damp and/ or wet where the water is entering. Keep us posted.