Different width or diameter?
Dunno about legalities, but a difference in rolling size; eg 31" to 33" when in 4wd could screw the
diff, due to tranny wind up.
What exactly are you thinking of doing, be a bit more specific if you could please mate.
it is not good for your truck to do put different size wheels front and back because in 4x4 the front will be moving at a different speed to the rear and could cause wind up in the transfer case and damage other componants
decide what you want to run, sell the others on ebay ot something.
I have 2 sets of tyres / rims.
31x10.5 on stock rims as the dailys, and 33x10.5 on offset rims for play, but if its one set you are after... thats cool, just decide what you want from them and what size.
just as "oils ain't olis"... rubber aint rubber either.