We've been up and down Priests Grade many a time on our way to Yosemite. However, there is a sign which either restricts or gives a gross weight and length restriction on the 'old road', and it's pretty low, like 9000 or 10000 pounds. Our TC is only 20 feet in length but over 10000 lbs. so we always take the 'high road' with the other overweight rigs.
The worst grade i've seen in CA is State Route 108 which goes over Sonora Pass and has some stretches of 24 to 30% grade. Many an automatic trannie has met its demise on this paved over cow path.
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regards, as always, jefe