Couple of issues.
First, a 225 tire is actually bordering on too wide for a 6" rim......235 is too wide.
If the rims are the stock offset, you may very well have clearance problems with rubbing the strut and/or the fenderwell lip. I can't say for sure, since no one that I know have ever even considered a 235/75 tire.....much less on a 15" rim.
What kind of tires are these, andwhat do they normally fit?? That is a very oddball size. Plus, if you have ABS and only put them on the rear, the difference in diameter can give you an ABS light and disable the ABS.
Bad idea, IMHO.