Hhmmm, let me state that I've never tried to flip a Jeep. But when rolling something, the entire weight of item isn't lifted. The weight needed to roll becomes less as the items center of gravity gets close to the corner.
The terrain would also come into play. Considering that the reason for the roll (hill, rock, tree) may still be there, that could make it harder.
I think on level ground, 2-3 normal guys could get a Jeep back on it's wheels by themselves.