From the East end of Tucson, I-10 @ exit 281, you turn South on 83 towards Sonoita, about 30 minutes on 83 you will be up on the elevated grassland around Sonoita. Turn right onto Hwy 62, at the sign for Greaterville.
This is paved for a few miles and you will come to a fork in the road, Box Canyon to the right & E Greaterville Rd on the left. You will turn left onto the gravel, still following the signs toward Greaterville. Driving up a gravel road, you will come to a 4 way intersection with a large open gravel area at 31*46'5.44"N , 110*45'10.95"W. This is the last place a regular car or low-clearance 2wd vehicle can easily get access to. After this you should be in a high clearance, 4wd vehicle to be safe.