So i'm in the beginning phases of planning out lifting my truck and I'm thinking about switching to a larger tire as well. right now I have 285/75/R16 Duratracs.
I have 4.0L Auto 4x4 with 4.10 gearing right now. I would like to go up to 315/75/R16 Duratrac's. Would this be okay with the stock gearing or should I think about going up to 4.56's?

 

the 315/75/16 is pretty much a 35x12.5 so you'd want to go with 4.88's not 4.56's.

I'd definitely regear both axles. It'll be incredibly sluggish without the regear

 

What this guy said!

 

x3. You'll defiantly want to regear to 4.88s. You will regret it if you don't. Your engine and transmission will appreciate it as well.