I have a 99 ford ranger 4 banger. it is a stick shift. my clutch is starting to slip due to the 31 inch tires its rolling on. does anyone know what ratio gears i need to replace the stock diff with?  

31's? hmm... with that engine 3.73's would work nice, or you could go 4.10 but that would be a little tall with only 31's...

Edit: Actually, whats in it now? If I had to guess I'd say 3.55 or 3.20... but I can't say for sure... let me know that and I can help more.  

Oh, and the best cheapest way to do it would be to swap an 8.8 out of another Ranger into it. Most all of them came stock with 3.73's.