Mono county (eastern Sierra) has over 30,000 miles of 4x4 trails and roads. Bishop, just a few minute south of the Mono county line, would be a good base as it is surrounded by trails and has the conviences of civilization. Inyo county also has plenty in the way of off road fun. A local expert would be John Crowne in Mammoth. Just ask anyone in Mammoth for him or stop in the Clock Tower bar and ask for him.