My truck has started idling really rough once it is put in gear. RPM's drop to around 300-400. Does anyone have an idea what might be going on?

Could be alot of things. Miles? Mods? Last tune up? Bad gas? Just start?

Yeah your throttle is gunked up. Get a can of throttle cleaner and wipe it out with a rag. Idle will pick right back up and smooth out. Lights will be brighter at night also.

Vernon

I will try that. It seemed strange to me that it would idle fine until i put it in gear then it would drop a couple hundred rpm and would be a little sluggish.

yea i have the exact same truck as you
it seemed to have happend when i changed to 93 octane fuel..didnt know if it has correlation
once or twice it cut off at a light

Clean the throttle body. Seen it many times.

Just cleaned the throttle body and no change. It still idles rough when in gear. So far I have changed the fuel filter and cleaned the throttle body.

Last plugs? Wires on good? Vacuum leak somewhere?

02-04 5.3 & 6.0 ltrs were known for leaking intake gaskets. Updated design seems to be holding well. Spray carb cleaner at or near the injectors for each cylinder and listen for rpm change. Or you might have a fuel pump going out.

The truck has 115,000 miles. The plug wires are on good, and i dont know when or if the spark plugs have been changed. I am going to check for a vacuum leak in the intake. Hopefully it is nothing too serious. Although this might help justify a duramax or cummins swap.:)

Doesn't appear to have a vacuum leak. I dont know what it is. I am taking it to a friend of mines shop. Hopefully we can figure something out.

I would agree with above. If it just feels like a rough idle and it smooths out once your running, it sounds like an intake leak somewhere.

You may also check your plugs. I just changed my 8.1L suburban's spark plugs at 95k and you could almost slide a nickel and penny together between the electrode and insulator on all my plugs. At least .080" gap on all my plugs.

If it's not that and you know you have good fuel pressure (pump and filter are both good), you might have a dirty/bad fuel injector. Does it feel down on power?