1999 Blazer 2door 4x4 3.42 gears
How do I determine which size differential I have and which locker/posi unit I need?
Are rear ends from Camaros interchangeable to the Blazer or just the locker unit?
How hard is it to install a G80 locker in my diff or should I take it to a shop? (Very novice wrencher with limited tools?

 

It's a 7.6" GM corporate 10 bolt axle. If you are a novice, it might be better to bring it to a pro. Could be easier and cheaper to install a complete rear axle from a G80 equipped Blazer instead of trying to install just the G80.

 

Yes you can swap a "G80" carrier from another s-series or 3rd/4th gen camaro ten bolt (assuming you don't have the larger 8.5 ten bolt). To do the job correctly at a minimum you'll need: a torque wrench, new ring gear bolts, the correct shim kit, a dial indicator with magnetic base, possibly new carrier bearings, and some marking paste if you want to compare gear patterns before and after.

Assuming you're keeping stock gears you should start by using a dial indicator to measure backlash before you take anything apart. Once apart swap your old ring gear onto the new carrier. Keep track of the bearing cap and shim orientation... Once you reassemble everything in the correct order it is very important that you get as close as possible to the same backlash measurement as prior to taking it apart. If it is not the same you'll have to properly add/subtract shims till it is correct. Remember if you add shims to one side you must subtract from the other side as to not affect bearing pre-load.

FYI
There are different carrier and posi types in the Camaros so make sure you get the right one. with 3.42's you'll need the 3 series carrier and id go for the torsen style posi from a later 99-02 4th gen.

If non of this makes sense, take it to a pro.

 

KM346 Yep makes NO sense to me!