Why is there heavy ice buildup in our 2 year old LG bottom freezer refrigerator model LFX25970

a bad defrost timer can be checked with a multi meter

Is this auto-defrost model? Usually, heating element that defrosts the ice in back of freezer can go bad. How old is the model?

Watch the following video on you tube. This may solve your problem.

You may have an issue with one or more of the following:

* icemaker
* dispenser actuator
* ice chute door assembly
* ice dispenser solenoid (if the solenoid is stuck in an open position, the ice chute door/flap would remain open allowing cold air to escape).

Contact an appliance service company.

Below is two links for servicing and owners manual! Hope it helps!

LG LRTB2023W Servicing Precautions LG LRTB2023BS Owner Manual

I don't think replacing the filter had anything to do with your problem.  If the ice maker is working then the freezer is OK since they have the same cooling source.  I had the same problem with this model refrigerator six months after I got it (in 2/2007).  The cooling in the refrigerator section failed.  The refrigerator fan and the air duct as well as the mother board were replaced.  A year later the ice-maker fan froze up. It and the housing were replaced with improved models.  The mother board was also replaced again due to software changes related to the ice maker.  This model is also part of an Energy Star/LG case agreement to modify the circuits and provide a lifetime annual rebate payment.  Check it out and register on the LG web site.

plug the water in

Does freezer back wall have ice build up ? if so a defective heater is the most likely problem

You have an air leak coming from the light switch in the ceiling of the freezer.  You will need to call LG for resolution on this problem.  I believe there is a fix for this problem.