Go the same size, so you can rotate them, providing that size won't rub up front.

 

The Tire shop is right, you'll have to use the same overall diameter when you use the 4x4.
You could change to other tires as long as you keep the same overall diameter front and back, left and right.
Use this link, where you can fill in you're tire sizes and compare them to stock, or to the ones you have on the back.

https://www.wheel-size.com/calc/?whe...cl=20mm&sr=0mm

If you gonna change all tyres you can pick any size you wan't for all 4, just as long they fit you're rims.
You have the standard 15"x8J ET25 rim, you can put minimal 215 and maximum 265 wide tires on those, you can go wider, but handling on the road will be less, for mud and terrain it's another story.
So buying tires is a lot depending on you're needs.

 

will a 97 Will the dodge Dakota fit a Goodyears Wrangler ST 225/75R6 Wheel?

 

will a 97 dodge Dakota fit a Goodyears Wrangler ST 225/75R6 Wheel?

 

Don't know exactly what you mean by this question ?

If i understand it correctly, you wanna change you're rims from 15 inch standard to 16 inch rims ?

If so it's no problem, as long as you check you're ET-value or Backspacing on the new rims.

If you wan't to change to the original Dakota/Durango 16 inch rims there's no problem at all.

For the tire size you're mention, i don't know how much room you have, consider the height off the tire 75 and you're wheel wells, i think if you are on stock ride height you should be okay.

The wide of the tire 225, is no problem, but i would not go any smaller/narrower, i run 225/70R16 on my original Dakota 16 inch wheels, but mine is lowered,those are my winter wheels.
During summer ive got Dodge Viper wheels with 255/55R17 up front.

 

When i reed you're first post you said the door sticker was 215/75R15, so they are 704mm tall and 215mm wide.

You now have 31/10.5R15, so they are 787mm tall and 266mm wide.

You wan't to mount 225/75R16 Right ? these are 744mm tall and 225mm wide.

So if you change to the 16 inch rims there are no problems at all, considering you had no problems running the 31x10.50R15 Wheels, you're new 16 inch wheels are smaller in overall measurements.

 

Are those new wheels six lug? If not, they won't work.