Is there a minimum range between RBSXTG-5HnD devices?

The site were using these is about 1.5KM distance between points and do have site to visibility with no obstructions.

Also, the first site is about 11 meters up the second 8 meters up. Is the height differential a problem?

Thanks.

Fresnel zone would be your only concern. But you should be fine.

http://en.wikipedia.org/wiki/Fresnel_zone