Unfortunately for you the stock GM exhaust on the 99+ trucks flows extremely well, minus the oil-drum of a muffler. That's the only real restriction on the stock exhaust. The rest is mandrel bent 2.75" tubing, and very free-flowing.
Frankly, you won't need LT's until you get a cam. You'd spend upwards of $700 for VERY minimal gains––think maybe 5-8hp at the wheels.
If you want more responsive driving, the best thing you can do to a stock truck is get a tune.
I made the mistake of going with Wait4Me––it's a nice upgrade over stock, but leaves a lot to be desired; if I had to do it all over again I'd give Justin at Black Bear Performance a call and have him do a data-logging tune. There's plenty of HP to be unlocked on the 5.3/6.0 from a tune alone––upward of 25-30hp WELL DOCUMENTED gains.
http://www.blackbearperformance.com/
For more power, remove the stock cam (which is an amusingly small 190/194 110lsa, or something in that neighborhood) and throw in a 212/218 114lsa cam (Voodoo's Lunati line is great), throw in some LS6 springs and some pushrods and you're well on your way to an extra 90ftlbs tq. w/ a good tune. Throw in a bigger stall like one out of a Trailblazer SS or Trailblazer 6cyl and get your stall up to 2600-2700rpm for more meaty torque off the line.
All this can be done for under a grand––Cam is $350 new on eBay, LS6 springs are $90 new, tune will cost in the $250 neighborhood, you can re-use your pushrods, and a stall can be had for $150 or so new or very lightly used from the Trailblazer folks over at performancetrucks.net.
...Or you can throw money into the fart tube of an air pump and effectively waste cash.
Exhaust is gimmicky––every manufacturer claims HUGE gains. Get what you think sounds best and run with it. Honestly you're not going to get much––if anything on the butt dyno with a muffler swap, even though the stock muffler on your Avalanche is akin to a labyrinth inside.`