I am thinking of lifting a automatic 4 door tracker would I need to regear it to run a 31 inch tire?

There seems to a big difference between 30" & 31" tires in how much power is required.  I think you will want to regear to get some power back.  If you have a 3 speed auto you have 4.30's and could go to manual 5.12 gears, but if you have a 4 speed auto to get a lower gears you will need to find the rarer ones.  And they are usually pricey.  31" tires also require lift, messaging &/or modifications to keep from rubbing.

Since you have a 4 door, you will have 5.12 axle gears (the 4.30s only came with 3 speed auto trannys and a 16V and the 4 doors never used the 3 speed...all the autos in the 4 doors were 4 speeds).
So you will need to find a set of 5.38 or 5.62 axle gears.  Yeah, they can be hard to find and are usually pretty expensive, when compared to the common ratios.  I do have sets of each in the classifieds section.