I used Eastwoods Rubberized Undercoating.
I bought 4 commercial cans(non-aeresol) and their undercoating gun/wand combo.

First, I pressure washed the underside very well the day before. It wasnt all that windy out, so I parked the truck in my grage and put a monster fan blowing front-back to dry everything out.
The next day, I taped and covered: driveshaft, bolt ends, transfercase/trans and then went to town on the underneath. I put 2 good coats on everything.

6yrs later and this is how it looks



Here's a pic showing my differential which I just took 3 months ago


Make sure you buy and wear the following:
throw away painters smock with hoodie
goggles
gloves

 

For the rear quarter on the box.get a Flashlight .. Take out the rear tail lights get a 4 x 1 inch roller, 6 foot stick to go on the end of your roller, gallon black rustoleum oil base Paint, plastic cover up the wheels and garage floor unless you live out in the country . I would do this in the back 9. Start mopin threw the tail light openings if you got a brush and some old gloves , do underneath the cab and touch up with spray bomb for the area you cant reach. Take the flashlight and look threw the tail light openings and look up from under the truck to see if you missed anything . As for the doors I dont worry about.. You could take off the inner door covers and access that way.. Mines 06 not blisterin yet..I prefer gloss black make the wheel wells shine..

 

Has anyone ever tried getting it undercoated professionally?? Like Zebart or something?

 

I prefer to use Por 15 (http://www.por15.com/) in the fender wells, you can hit that stuff with a hammer and it won't come off. I also heard that Eastwood has some great products as well. If you really want to go all out, get a tonneau cover or bedliner. I got one from DualLiner (http://www.dualliner.com/dodge-ram-bed-liner.php) and noticed a difference in the water collected in my bed versus rusting out rest of the truck.
- sorry wouldn't let me link anything.?