My 99 2wd Yukon Vortec 350 with 4l60e was getting 14 hwy (75-80mph)around 60,000 miles.
I put in a b&m shift kit, K&N cold air intake, and dual 2.5" cat back with a dual in dual out 50 series flowmaster. At the same time I noticed a few pulleys slightly out of round and it turned out to be the tensioner and the crank pulley.
With all that plus a new intake gasket, waterpump, and now running mobil1 in the oilpan I brought it back up to 20 mpg with 94,000 on the odometer. Not bad for a 6,000lb suv with 3.73 gears I think.
If you aren't sure of your gears I have the rpo list on my site HERE
CLIK FOR RPOs