If you're asking if you need the front diff as well, then you will need to know your current gear ratio. If you currently have 3.73 gears then you do not need to swap the front diff as well. If you have any different gear ratio then you'll need to swap your front diff as well to match the rear diff ratio.

I have 3.42 gu8 rear end. I have been searching for a zr2 donor

So yes you will need both rear end and front case for the swap. Ratios need to match front and back.

I guess 3 42 will do for now

I'm in the same boat, but I think I'm just looking for the rear axle. I've debated on getting a parts vehicle if I ever come across one, I know I can take it apart and make my money on part, let alone the parts I would use.

Maybe the servo upgrade will give me a little pep