My 2000 Jeep Grand Cherokee overheats in traffic

Check the fuse for the fan and use a test light to make sure that the circuit is functioning. Warm up your car then feel along the top and bottom of the radiator to check for cold spots. If you find any then you may need to replace your radiator. If your heater isn't currently working it may be that since it is a part of your cooling system.

I had the same problem with my Jeep Cherokee Laredo and I replaced the fan motor.. problem solved!

You said something very important in your post. since you replaced the controls for the fan, have you checked to make sure the fan itself is good? You can easily test that by hot wiring it directly to the battery. If it fails to operate, replace it. Also check the outside of the radiator for obstructions such as bugs or leaves packed into the A/C condenser in front of the radiator or leaves stuck between the radiator and the condenser. If the radiator is internally blocked it is difficult to detect as most of the radiator is not accessible most shops use a heat sensing gun to check that.
If you decide to flush the system, flush the radiator separately from the engine as internal rust from the block will pack up in the radiator core and make the situation worse.
Another item to check is the clutch fan...It should not freewheel when the engine is hot.
The reason that it overheats in traffic is that in reality when at speed you don't need the fans...air flow caused by the speed of the vehicle is sufficient to keep everything cool. When in traffic you need fans to create that "wind" and any obstructions to the radiator become far more critical as Jeep radiators are borderline too small for the system in the first place.
Refrain from driving till you fix this as you can cause far worse problems (blown head gaskets) if it runs hot or overheats too many times.