I read that having diff size tires in the back can ruin the differential is this true?

 

Depends on the type of diff but yes

 

I assume you mean different diameter and widths left to right side in the rear? It would not be wise it would make car sit funny and drive weird. Diff would constantly be unlocked if a posi. Trying to lock both up would lead to one dragging the tire since it wont spin same speed

 

You want to make sure you run two tires of the same size on the rear end. They don't have to match the front two, but the rear two should be the same size.

 

That's what I meant I have 2 225 55 r16 and in the back 2 235 60 r16

 

If my quick math is correct then it is about 1.4" difference in height. Make sure they are in pairs front and back

 

I agree with Midias. As long as you keep them in pairs on the back you are ok (i.e. the 2 235s are on the back)

 

I always see people with bigger tires in the back as long as they match

 

Right, I'm one of those people. It's not going to tear up the differential, no worries. (I have 275/60/15's in the back and have no issues.)