To go from Isiolo to Moyale (the Ethiopian border) or the other way around, the only possibilty is the lorry.
You can talk to truck drivers in one of these cities to hire your place on the vehicle: either in the cab or on top of the truck (the former being more expensive than the latter).
From Isiolo up to Moyale, they usually leave early in the morning. (We left at about 4 am )
It's not very comfortable, but if you are young and fit, it is worth it: the views in the beginning of the trip, from Isiolo to Marsabit, are impressive: mountains, desert,... and here and there people living in this remote area's, or people herding their cattle.
From Moyale you can also hire your place straight to Nairobi.
We fortunately had a driver and borrowed the bishop's white Landrover. We needed this for not only the reserve and the park, but even the town streets. The streets are generally not paved... full of potholes and the drivers are a bit crazy, like most of Kenya. You just need to keep your wits about you. But another thing about Landrovers. You can get either a lot of people, or a lot of luggage. We had 6 people in the front and middle, and Dudley sat in the back with the suitcases. What a man with a big heart and lack of claustrophobia... but then in Africa, you lose your sense of personal space. If you've ever travelled in a matatu, you would know what I mean!