Lobo
I used to have an aquaintance (friend of my father's) who did it for us... but even he had a special setup. He had a hoist, and a pressurized spray system, and of course, the proper kind of oil. He worked Saturdays doing this for all kinds of people. Last year I found a coupon for a different place, and since it made it the same price, I thought I would try the "professionals", and see if there was any difference. Well, the pros drilled a few more holes, in some different locations (holes for oiling inside the body). So it appears that they know what they are doing a bit more, as far as where to oil in any given vehicle. In your case, all I would do is look up "Rust Proofing" in your phone book. But many places will try to sell you different things and offer warranties (if your truck is new enough.) Just try to find someone that does "drip and/or semi-drip" oil spraying.
As far as doing it yourself goes, I suppose one could... but you'd have to research on what the proper kind of oil is, and where to get it (Note: Motor oil is not the right oil, especially used motor oil). You would also have to have a good way of applying it, so a pressurized spray that emmits almost a fine mist or something. And you would have to find out where the best places are to drill inside the doors. Also you would need the rubber plugs that they put in the holes when they are done. It costs me around $40 or $50 Canadian, so I think having it done for you is the best solution. Best of luck.
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JGB - Ontario Canada
'96 XLT 4x4
K&N, Bosch, Splitfire...