you need all 4 wheels aligned mate, if you change the rear you may have fitted them up wrong so that the rear end isn't the same as it was

all sorts of things to look out for but you won't be able to measure thrust angles and ackerman steering etc at home with a ruler and a bit of string

if you can find a place they normally cost about 350-60 quid for all 4 wheels to be done

make sure you get a print out of what the before/after angles were to be sure things have been done