Fan does not blow hard out of vents - 2002 Jeep Grand Cherokee

Do you have an AC pollen and dust filter? If so it is likely plugged and no air flow is getting through. Another cause of this is that the AC condenser core inside the dash is plugged with dirt and leaf matter (cars without any filter) I really need the make, model, year to be more precise.

Make sure your vents are fully open.

you mean low heat. at all ducts with full fan speed and blows hard,
here is the correct order.
1: engine at 180F or more holding, a absolute must.
2: fan runs at full speed on HIGH, (sounds like it)
3: fan actual blows (felt) at full speed, to any vent, try all vents.
4: now heat, will happen, if not, the dampers are closed or wrong.
damper can be complex on newer cars,they have motor controlled damper servos';

is this just blower fan dead? problem. on high, if yes say so.

may be something plugging the heater tube itself, maybe a rodent has set up camp, fan resister works both cold and heat fan speeds, unless they have a resister for cold and hot which I have not heard of.

If the fan is not blowing on high you would check the resistor and for blockage in the vents. Could also be a blend door problem.

If very little air comes from the vents, the fan on the blower motor may be damaged, or there may be a restriction in the vents, or a blend door may not be open completely.

you may have a cabin air filter. check your owner's manual for service directions. Or the air inlet is clogged with leaves or other debris.

check for interior filter if it has one or the air door actuator for broken linkage to air door under dash left and right sides

check vacuum lines under hood for leaks

Have the blend door and the resistor checked.