The best way to remove exhaust manifolds on these trucks is very carefully with lots of PB Blaster or some other penetrating oil. Soak the studs with penetrating oil hours before you plan to remove the manifolds and then turn the nuts very slowly and methodically. The exhaust manifold studs often stick and it is very easy to break one off in the head. For that reason, you should avoid using an impact wrench. Also, it is easier to reach the manifolds on your truck if you remove the front wheels and the front plastic wheelwells.