my car came with the GU2 G80, I want to swap out the gears to 3:23's when I change out the ring and pinion if I lay the spacers out in the order I removed them. Do I still need to have the rear end setup or should I take it to a shop and have them do the swap for me?

 

GU2 gears(2.73) came on a 2 series carrier. 3.23 gears will not work with a 2 series carrier,you would need a 3 series carrier. The only gears that'll work with a 2 series carrier are 2.73(GU2),3.08, and aftermarket 'thick' gears of 3.42,3.73, & 4.10.
Whenever a gearset is changed,pinion depth and set 'lash' has to be set. Any shims/spacers(pinion & carrier) from the current gearset are only a starting point in 'setting up' the new set.