My 1991 3.3 dodge van have a low speed miss new plugs/wires,. runs good at high speed,what I need to do?

Oxy sensor or timing. Check timing first before you start replacing parts. A bad exaust might also cause this.

If you haven't done so already, I would replace the distributor cap, rotor, spark plug wires, spark plugs, and possibly coil. Ensure spark plugs are an exact match to what originally came in the van.