In regards to Versa-Loks wall diagram on low walls, its safe to build what is refered to a gravity wall up to four feet in height with complete granular backfill.
That's if there is very little sur-charge behind the wall and that it is level in front of the wall.
Under your conditions, with the slope that you discribed, it would be best to put a layer of grid half way up the wall. Best grid to use for your conditions is a bio-directional grid, that are usually 4' wide by 50' lengths.
The grid will actually stabilize the soils behind the wall, thus creating a minature failure plain, instead of a large failure plain pushing on the base of the wall. Hope this helps.
Soil Identification & Sub-Grade Prep