Do i need to remove the entire gearbox to install these gears or can i just remove the tranfer box section?

with a manual, you pretty much need to drop tranny and tcase.
with an auto, you *may* be able to get to the top bolt (especially if you have a BL).

I've never dropped the trans, only the t-case. The top bolt is a pain but it is not that bad.

I've never dropped the trans, only the t-case. The top bolt is a pain but it is not that bad.

I would agree with that, unless you don't have a 2" or 3" body lift.

I was able to remove the t-case off my tranny without too much dilemma, and i have a 2" body lift.  I don't think there would be enough room to get in to those top bolts if you didn't have it.

I've done it with and without a 2" bodylift on a manual. The bodylift makes it easier. Autos are even easier though.

Remove the cross over bar holding up the rear of the trans and let the trans hang.  This will give you a little extra room.  Remove the shifter (make sure its in neutral) and all the boots.  Now the tricky part. remove the 5 bolts for the shifter linkage.  Under that is the last bolt holding on the case. 

When replacing the case leave that bolt out and the next time will be easyer.

Here are some photos to show you those bolts.

Click on them to make them HUGE.

I have the 2" body lift so that should make life easy.

How will the will the vehcile perform with these lower gears compared to stock?

You will love them.  The only problem I've ever had was that they are too low for sand.  But I mostly drive in 2wd in sand anyway.

Good luck