Hard idle at start

I live in WI, been having some colder nights lately, and so began my problem.
 
2002 Silverado 5.3 V8, 94k miles

Heres the deal:
Start the truck up in the morning, turns over fine, revs up, comes back down to idle and idles hard. Sits at about 600rpm and every second or so dips down momentarily to 400rpm, almost like it's losing power or a miss fire. It will do this idling at the first few stop lights on my drive to work, no 'misfire' while actually under-way. After driving for a couple minutes and motor is warmed up, problem goes away. No problem leaving work. Service Engine Soon light only comes on now and again, but not consistant with the problem, though. I had it scanned at AutoZone and it came back P-something, the guy said a tune-up code, possibly wires, plugs, or distributor.

Had it scanned again...
6 codes, each one came up twice?

P0300 - random misfire
P0171 - system too lean
P0174 - system too lean

A counter guy at Checker said his Ford had the same type of problem but would idle bad all the time and turned out to be a vacuum leak. My brother thinks something with a dirty o2 sensor or gummed injectors.  Also heard on another forum a possible intake gasket leak.

So, any ideas what could be causing this before I take it in to a shop and drop some dead presidents on diagnostics?

Welcome to the forum!!! First I would start with the cheapest fix (gummed up injecters), you should run a cleaner thought the system, I would suggest BG 44X, really great stuff (cat safe). you can get it at:
http://shop.autobahnpower.com/products.asp?category=BG%2044K%20and%20BG%20Products&product=BG%2044K%20fuel%20injector%20cleaner
Second I would probably do a tune up. How long since the last tune up?
If that does not fix the problem then I would move on to checking the vaccum lines, then O2 sensors, then Head gasket.  

I have some injector cleaner running through right now, so I'll see if that clears it up with in a week.  I'm not sure about the tune-up as I just bought the truck in November.  Yesterday morning it started and idled fine, but the overnight temp was pretty mild for this time of year.

You can pull the plugs one at a time and see how they look, if they look bad then its probably time for a tune up.

I had/have a very similar problem on my 97 explorer.  It started out just like you described until eventually it started occassionly dying at stop lights when the tac would drop.  In the last two years I have taken it to several shops and each one had a differing opinion.  I replaced the o2 sensors (per an AutoZone scan), replaced a fuel pump/filter, had a tune up and used injector cleaner also replaced several electrical parts including the plug wires.
 
The last guy I took it to was POSITIVE that the problem is a head gasket leak, but I haven't had the chance to replace it.  Best of luck.  This seems to be a very common problem w/ late 90's early 2000 explorers at around 100k... Might not be a bad idea to troll some ford boards.

if the truck has 94K on it, chances are that it is well over due for a tune up. car dealers do not tune them up when they sell them.
when your in there, make sure you clean the throttle body real good with some intake cleaner and even scrub it down with a rag

These symptoms are normally caused by a leaking intake manifold gasket on the 5.3L.

Is it primarily the upper or lower gasket, or both?  Any idea on the time frame/cost of this fix?

The gasket(s) that I am refering to are the intake manifold to cylinder head gaskets.
Are you planning to make the repair yourself?
Most dealers charge about $600 or so for this proceedure.

Possibly, yeah.  My brother is an auto tech, he's replaced a few before.  I heard they're not too bad of a job on the 5.3L?

A good mech can do the task in two hours.