well, what you have in the rear depends if you have an auto or manual and if you have a 4 or 6. if you have a 6 with a manual then you have 3.07, if its a 4 with an manual it is 4.10. (or 4.11 cant quite rember). Look on the little tage on the axle to see what it is exactly. Running 31's wont make your spedo to far off. i think i was 5-8% off. what you can do is when you get the tires on, go drive exactly 10 miles measured on the highway (this gives better measurments but you can do 1, or 5 or whatever) and check how far your jeep says it is on the trip guage.
Its not what you buy...its what you BUILD!!!
1995 YJ ,SOA with 4" procomp front 3.5" BDS rear 1" shackle rear, custom stearing, 35x12.50x15 pro-comp MTs, warn xd9000i, on-board air from A/C, dana 44 rear and 30 front with 4.56, powertrax no-spin rear, custom disks rear/trutrac front, warn front shafts, HD SYE kit, HD front and rear driveshafts, Warn 4X flairs, PIAA 520 ligts and on and on and on :)