I was wondering how do I tell what size my rear diff is I am gonna be putting new gears and an LSD in, I know I have the GU6 gears right now was wondering what size to put in

 

Unless you have a ZR2, you most likely have a 7 and 5/8 rear (or 7.625). As far as gears, I have researched that 7.5 and 7.625 are interchangeable, but the 7.625 are quieter because they have tighter tolerances. If you have 3.42's now, what are you planning to change to? What do you want to accomplish by re-gearing? 2WD or 4WD? Once we have more info we can answer your questions better.

I am in the process of putting a G80 (supposedly) into my truck with the same 3.42's. If you plan on offroading a lot, there are better units out there. Problem is better=more expensive.

 

I'm looking to change to some 3.73 or 4.10 gears looking for acceleration really, its a 2 dr 4wd but I don't go off road just use the 4wd in the winter pretty much, I'm also in the process of putting a 100 shot of nitrous in the truck .. but who knows I might not even change the gears, I am looking at rebuilt g80 units also most likely I will prolly just end up doing that since the ring gear set for my truck is around 560 although I can prolly get it cheaper since I work for a auto parts and body supply shop.......

 

the G80 won't be much help to you at all if you're not going to take it offroad much. Not sure how familliar with them you are, but it is a gov-lock style, so once the speed of the vehicle is over 20 mph, the locker will not engage. Imagine what would happen if you were goin down the road at 50 mph, and one of your back wheels went into the dirt on the shoulder...the axle locks up...and you've got quite a spin as it matches wheel speeds...

Now you can modify the G80 and cut off the weight that governs the locking mechanism....I've seen it done on some Volvo's that see some track time.

And if you wanna hook me up with some 4.10s....