Dutchmen Sport,
Do/did you drive the 2005 on gravel roads a lot or high clay content roads? Sounds like you have salt containing wet clay dust built up along the door bottoms. You can't erase the rust but you can stop it unless it true rust thru but will even slow that down a lot. How many miles on the truck? Under 250-300K? Being a diesel it's likely to go another long ways and years before any major issues. Perhaps it's paid for and keeping it for a few more years and putting the amount or most of it for a truck payment in savings may make a new purchase down the road almost painless. Having a new truck in a few years with little on no payments and virtually paying no interest is beautiful!
For your 2005:
High pressure spray to clean the joint as deeply best you can, then dry thoroughly with heated air. Spray WD 40 (that's Water Displacement formula 40)so it fills the cracks and replaces any water still inside. Then dry very thoroughly with heated air untill really dried out and wipe off any residue. Next, spray Rust Fix or the equavilent using the little plastic tube supplied so you can control over spray into all the door bottom voids or cracks you can find. Maybe some areas will need 2-3 coats. Let dry and turn black and then if desired, you can coat with a Dupli-color etc matching spray so you can't see any rust color or off color spots. You'll likely get another 3-4 years before rusting starts again. Still can do it all over again too for more good years! How did you crack the wheelwell trim or bumper facias etc. Cracking is not normal unless it's hit.
Our 2004.5 Chevy 2500 D/A CC LB 4X4 doesn't even have a hint of rust anywhere as I constantly check it. Looks nearly showroom new! Almost 150K and lots of unpaved road driving but I do make sure all body vents are always clean and open so trapped water will drain and I also flush in spring to rinse the salt away. Trucks are way too expensive not to and so are cars!
