If you want to keep your current engine, a larger exhaust including the manifold (go as big as you want, there is no valve overlap on the diesel so you don't need back pressure), a rebuilt pump (Giles is the man to talk to) and new injectors (also talk to Giles) will make a huge difference. Make sure to change your timing belt ASAP and have it properly timed.
A cold air intake will also help you out, as will synthetic motor oil, fresh tranny fluid, no power steering, A/C delete, good bearings, no dragging brakes, a large duck bill (air resistance) and lowering your truck. Everything in this paragraph will have less of an impact then the first paragraph, unless for example your bearings are shot.