Maytag MBB1954GEW fridge does not cool

11 year old fridge with a bottom mount freezer no longer cools. The top compartment is warm. I don't feel any cool air movement when I put my hand against the grills in the top compartment.

Freezer runs fine but never stops even on the lowest setting. The coils behind freezer back panel are solid frozen. I thawed the coils completely a month ago. The fridge worked fine until the coils froze up again.

I searched and found this older thread:

http://www.applianceblog.com/mainfor...r-cooling.html

It's the same fridge model. The problem sounds very similar. That thread mentioned three parts to check:

the defrost timer
the defrost thermostat
the defrost heater

Is it possible I need to replace something else, a more expensive part?

I'm trying to figure out how much the repair would cost if I call a repair guy.

Thanks!