How much extension do you need in the driveshaft?

I have a 00XJ, auto, 231w/PORC SYE. I've been running at about 5" lift and using a front driveshaft in the rear with no issues. I just put some 5.5 RE colis and 4.5 Leafs on so now I'm over 6" right now. (I know it will settle some)

My question is how much extension is recommended on the driveshaft? I have about 3/4 " left to extend when the vehicle is static on the ground.

You are at great risk of pulling the driveshaft apart when your suspension droops with only 3/4" of splines engaged. You should get a longer shaft before you drive it IMO.