Join Date: Dec 2010
Location: The Great Big Box
It will last quite a long time on 31" tires as long as you have a light foot. 35" tires is going to put way too much stress on the D35 rear, even just rolling down the road. An 8.8 swap will set you back between $300-$500 depending on where you source parts, maybe even more. Re-gearing isn't cheap either, it will cost you anywhere from $400 to $800 depending on who does it, and that's per axle. The cheaper option is to find stock axles that already have 3.55, 3.73, or 4.11 gearing in it and swap them right in. You should have 3.07 as stock.
I would list your tires up for sale on Cragislist or here, one of the local offroaders will probably go for them. Maybe even trade you straight up for a brand new pair of rubbers in a 31" variety. Removing the shackles is a good idea, you can run into issues with shackles that long (pinion angles, vibrations, etc.). A pair of 1" boomerang lift shackles can be had for around $100 a pair, and trust me, you want to do it fast. Plus the 31" tires won't rub at full spring flex, those tires are almost guaranteed to.
Also, keep in mind the speedo might not have been corrected. If it reads 55mph then you might actually be going somewhere near 70mph.
All about that unibody life.