GT4 and G80 here.

 

Mine is still stock. Open GU6. I was told that the max tire size I can mount safely for it (for extreme off-roading use) is 31". If I go bigger the Blazer will become slow and feel weak too. Currently I have 265x70/15 (roughly 29.6"x10.4"/15).

 

GT4 (3.73) in my 4x4 with a G80....i have a ZR2 axle with a G80 that i have to get swapped in, i am just waiting until i can buy a e-locker for it. i have all the rear brake parts sitting in my bed room.... its starting to look like a garage in here.
my work blazer has a GU6 (3.42) open.

 

gu6 3.42 open.

 

I've got the 3.73 with the G80 locker, but, sadly, its locking days seem to be over.

Someday, though, it will be able to go to axle retirement, and a freshly built Dana 44, complete with ARB air locker and 4.11 gears will take it's place.

 

Did your gas mileage change any with the ZR2 rear swap?

 

Yeah, my mileage was hurting with the 31" tires and the 3.42 gears, especially in town and city driving. I gained about 1-2mpg (mixed city/highway) when I went up to the 3.73 gears.