With that kind of mileage...... plenum is highly likely to be blown then. The residue and such kinda indicates that. You probably haven't had the truck long enough to know if you have oil consumption problems....... but, that is another good indicator.

So, what I would suggest..... Get the plenum plate from hughes, intake gasket set from your local parts store, also a set of intake manifold bolts... ( they are torque to yield, and much debate if they are re-usable or not, better to be safe, and replace them) new timing chain and gears, and also water pump, and bypass hose. (cleverly hidden under the A/C compressor.....) Also, a complete tuneup set. (cap, rotor, wires, plugs), and get a PCV valve from the dealership. (aftermarket guys don't necessarily have the correct flow rate, and can suck oil.....) Do it all at once. (tune up is easiest with the manifold out of the way.....)

My guess is, your truck will run MUCH better.