[JK40]

Holy carp! Wonder what kind of axle is on that thing?

Probably Rockwells.

 

[JK40]

Sorry if this is in the wrong place or has been asked before, I couldn't find it on my phone.
I have a 2010 Wrangler Unlimited Sahara with a 3.5 inch lift, it's on 35's now. It has stock fenders, would I be able to get 37s on it? Or possibly even bigger? Or are 35s my biggest option without a different lift?

If you wheel at all, stick with the 35's. It's not just a matter of what will fit, it's also a matter of flex. E.g., the tire must fit when they moves up into the wheel well.