Uneven tire size on spare, how far can I drive?

I got an old truck with an open diff.

One of my fairly new rear tires got punctured. My tires are 235/75/15 my spare is 215/70/15. Drastic as that size difference is can I drive it 25 miles to the closest discount tire so I can use my warranty? Or will it fuck my differential up?

FYI spare has to go on the rear wheel because my front rims have the manual locking hub fused to it so they dont fit on the rear axle.