Long range xbee point to multipoint solutions?

Sir ,

In my project I need to create a - point to multi point zigbee network (two way link) .
The individual node are arranged linearly 2 k.M apart from each other. The maximum distance between the end device and the coordinator is 8 K.M. , The frequency of operation allowed is 2.4GHz ISM band. In each node some toll tower (10m height) is arranged. So please suggest the best suited xbee module in my application. Also micro controllers are available in each node and xbee should able to interface with this controller.
Any external antenna is required ? The project required high amount of redundancy so suggest the best fit solution.

Expecting your reply.
Thanks and regards

Rajesh M.

Do you have the ability to add routers to obtain this range?

What country are you looking at deploying this in?