5.3 engine running odd...

[70rs]

I have a 2000 Chev truck. auto trans, 4x4. All stock (exc K&N cold air intake tube) 150k miles.
A couple of days ago it started to miss at freeway speeds under light load/cruising. Today it backfired through the intake I think. (sounded just like a carb backfire). The miss goes away (at least I can't feel it) when I speed up quickly.
Can anyone give me an idea where to start looking for a problem? I am new to the whole LS thing. I kind of miss carbs and distributors right now.
Thanks for any help anyone can give!
Eric