Have you inspected the vacuum lines for cracks or intake tube(accordion section) for cracks, you could be getting non-metered air causing the engine to surge like as the ecu adds/reduce fuel. Can you repeat the symptoms while stopped, or only with the engine under the strain of moving the truck?
1200rpms is also pretty low to be driving around in, in any gear. You are well below the beginning of the 22re's power band, and your fuel mileage is likely suffering for it as well.
