86 South to S22 West (cheap gas, and a dump station at that intersection, btw...). Make a left on Borrego Valley Road (at the school), and follow that around to Yaqui Pass Rd (S-3). The part just after the top of the pass is the steepest part of this route, but take it slow, and you'll be fine. Take a right on Hwy 78 at the bottom of the hill. Take a right on S-2 at Scissors Crossing - DO NOT CONTINUE on Hwy 78 - Very steep uphill and winding. Follow S-2 all the way to the T intersection with Hwy 79 and make a left. Follow that until you get to the town of Santa Ysabel, and make another left onto Hwy 78. That'll take you right into Julian.
Enjoy the pie at Mom's!