Is it a good idea to rust proof my Jeep XJ before putting a lift kit on?

I would recommend getting the lift first, so that you get everything covered.

I agree do it after unless it's already rusty then do it a couple weeks before you do the lift it'll make it messy but much easier

I would do the lift work first then undercoat. It's messy stuff

On my opinion it doesn't matter because they get into any open spots

Yup. Do it after.

I would do it after, so they can spray everything on lift kit also. Also call and ask Joe the owner, he is very helpful

Do it after

Yes, but you might want to put the kit on first.