Hair over time can often change, sometimes it's the colour or even the texture that alters. If the hair is drastically darker in the back of the head then the simplest method of evening up the overall colour is to treat the darkest hair first. Also using a different product in this area to lift the dark hair would be advisable.
Speak to a colour specialist and have an in depth consultation, make sure they understand that you are unhappy with the difference in colour underneath and you would like it to be an even shade all over.
Hopefully this will solve your two tone dilemma.