I switched out the stock 235/15 tires for 31's on my 2000 sport. I'm guessing I have the 3.07's but not sure. Do I need to start thinking about regearing? And if so, what should I go to for ratio? Thanks.  

Not needed. Is it an automatic?  

Five speed. Thanks for the response!!  

If you clean off the gunk on the top I think RHS of the diff at the front you'll see the ratio. Regardless, if you don't feel any difference in the drive feel, then there's no reason to swap. If there was a difference, you'd feel it. Going from stock 215s to 30" on my X virtually dropped a gear in effect with 3.07s I was using 4th al the time on the open road where I could use overdrive before. Changing to 3.54s has gone the other way, but moving up to 31s one day will get things a little better again. Depending on the tread and stiffness, there should only be an inch or two difference in diameter between the 235s and the 31s.

If you were sitting on a wad of cash you could swap to 4.10s since you have an overdrive. But you really don't need to. Or if you don't have the Dana 44 rear axle you could keep your eyes out for some inexpensive four-cylinder axles (dana 30 and 35). They already have 4.10s in 'em from the factory and are often less expensive than a gear swap. I'd run 4.10s with the 31s-32s. 4.56 for 33s and 4.88s with 35s on your current powertrain setup. But I also do a lot of high-speed driving. If I lived more in the mountains I'd bump it up a notch or so.

What would be a good gearing for 31's on the 2.5L? It's the AX-5 tranny, 97 TJ.

4.56 would be nice but it isn't really a cost-effective gear swap. If you have cash blowing in the wind go ahead. Otherwise suffer through it.  

OK would you also recomend that on a ZJ with the 4.0? I have 31's now but want 32's or 33.  

The 31's are Interco TRXUS on Rock Crawler black steelies, 3.75BS. At the same time I put on E Bay sourced Rubicon shocks and springs {100.00} shipped, and 3/4" poly spacers and nothing hits,rubs or scrapes.  

I ran my stock 3.73s with 31s just fine.

the rubicon runs 4.10s with 31s stock.

I'd say 4.10s if you might go bigger