1990 4x4 Pick up sluggish and surging

I recently bought a 1990 toy pick up 4x4 due to the unreliability of my

1980 CJ 7. Anyway, I changed the plugs, wires, k&n air filter, and muffler(not the converter though). The truck starts fine and idles, even revs up smoothly with no load. When driving at lower rpm's its sluggish. At higher rpm's the engine surges. I bought a new fuel filter but i haven't put it in yet. I know this is a common topic but could someone help me out. Im new to the toyota thing. thanx JZ