tyres- replace one or both rears guys?

[Auuteage]

Got a tyre with a nail in it on my ute. I carry heavy loads every day and got to get it replaced as quick as possibly, no hanging round for a hour waiting...so im at the mercy of a tyre place in respect to $$, no time to shop round etc...

Anyway, can I get away with just replacing the tyre with the nail and have different treads, or should I do both? What are the pros and cons?

And lastly, approximately what would I be looking at for a 17 inch tyre of middle of the road quality? thanks :0)