15x8's are ideal for 33x12.5's from my understanding.. You realy don't need to go with 15x10's, but it will work of course..
You shouldn't have to worry about regearing since you already have the 4:56's, best bet is to put the 33's on and see for yourself.. Im running 31's on 4.10's with 22re and I have no desire what-so-ever to regear (Overall ratio should be the similar with 33's on 4.56's)

93' Toy xCab
bobbed > 22rEB > w56 > stock duals > SAS > 4.88's > 37's