The 12 is the rim size you could bump up to 25 10 12 back 25 8 12 front no problem but need to do all 4 b/c its 4x4.
You could go 23 11 12 , 23 12 12 or even 23 9 12. The 1st number is how tall the tire is next is how wide and last rim size.
1st two numbers you can play alittle with.
Look like you just want 1 tire so try and get the same tire or style you have on the other side.