Your model number is incomplete.
Kenmore/Sears numbers are usually 3 digits, a period and then an additional 8 digits.
Please check it.
Find your model number - AppliancePartsPros.com

[COLOR="Blue"]Why did you replace the heater?[/COLOR]
Your posts suggests that you had no heat at all but does not actually say that.

[COLOR="Blue"]How do I know which one to replace, or do I replace them all? And is there some way to Know if this is actually what’s wrong with it?[/COLOR]
Yes but we need a model number and then with luck we can find the wiring diagram for this unit. You can then troubleshoot the unit.

I would try the following first.
Poor venting is the most common cause of poor drying.

Unplug the unit and disconnect it from the vent system.
Now try a run.
If it now dries OK odds are the vent system needs checking/cleaning.
Be sure to check that the louvers on the outside of the house open correctly.

If you do not want any lint in the house.
Take a pair of pantyhose.
Put one leg into the other and then attach this to the dryer's vent.
Leave enough room so the pantyhose can balloon out like a windsock.

Both the above will let you check the temperature and the air flow.

Note: The above is not recommended for gas dryers due to carbon monoxide concerns.

Check the cycling thermostat on the blower housing next to the thermal fuse that typically would be the problem......it should be 4 prong.....2 prongs ohm test at infinity and the other one has low resistance........continuity test this i bet it's your problem....even sometimes when they test good they get loose inside shake it see if it rattles..........here's your part:
Whirlpool 8318268 Dryer Cycle Thermostat - AppliancePartsPros.com