Wikipedia has an article on Gerenuks which includes both species of Litocranius.
In Litocraniums sclateri, the head is larger (males average length 248-259 mm, females 226-231 mm), other cranial measurements are also much larger than in Litocranius walleri . It does not have a band of reversed hairs. It is more northern in its occurrence, liiving from the Berbera region west to just over the Ethiopian border and in Djibouti.