This is VERY common on the 12v engines. What is happening is the compression of the engine stopps the harmonic balacer faster than the belt and the subsiquent accessories (ac compressor, alternator, p/s pump) So you get a squeek. Nothing major. Guys have found on the 12v engines that the Goodyear Gatorback belt helps get rid of it.
Don't know about yours, but when I got mine new there was a tag hanging on the cigarette lighter with info on the engine. One of the things that it said was you may notice a squeak on shut down. No harm, no foul, no BS'ing. You'll notice it more in the summer months if you have the A/C on at shutdown. Just the high compression stopping the engine faster than the belt can stop.
This may be the only time I have heard of the dealer telling the truth Yeah, its normal.....mine has been doing that since day one. Its funny to hear all the powerstroke guys say "hey, whats wrong with your truck?....mine doesnt do that." I just grin and reply...."exactly"
I remember one time whn I pulled up to one of those side of the road car washes and when I turned it off, one of the girls jmped 2 feet back. One second I was laughin because of it... then the next I was frowning just because of how annoying it is. I know of a few that have gone out and bought a replacement belt that has solved the problem.
That shake is exactly why most guys dont run fiberglass hoods. The "death rattle" can cause cracks at the mounting points. I was going to install a functional scoop on mine but a friend that does bodywork said I'd be wasting time and $$$.