Using a tire calculator like this one. http://www.4lo.com/calc/gearmetric.htm
Says your stock tire is 27.7 x 8.5in.
And the 235 is 28.9 x 9.3.
So that is 1.2in x .8
Get a tape measure and add .6in above the tire and see if you got the space. And see if you got .4in on the inside of the tire.
My guess you could easily fit even larger tires.
As far as Walmart not letting you get larger tire, I'm not sure. It does sound like something they might say, due to them not wanting to be liable for going beyond stock requirements.
There are tons of tire shops you can go to that WIll get what you want.