Does any one know the widest tire I can put on a 91 stock 4 runner rim?

 

You can put 32x11.50 on it but it will rub at full turn. I've seen 33x10.50 on it also.

 

It's not so much the size of the rim it's the insane 4¾" backspacing that kills these rims. You should be able to fit a 33×12.50 or 35×12.50 if not for this. Change the offset (less backspacing) or use a spacer and a 12.50 tire will fit just fine.