I had to go up a 30-40% grade Saturday night to get to where a friend of mine got himself stuck in his jeep, he got his transmission crossmember hung on the crest of a steep hill with the wheels mostly off the ground on both sides. Threw it in reverse, let the clutch out and he popped right off it...
truck did fine, didn't feel tippy at all. I'd be more worried about side hills than going straight up a steep grade.
EDIT: I am a dumbarse and actually meant 30-40 degree. Coffee first, THEN post.