Hi: Thanks for the replies and I do apologize for not including the model info on the original post. I should know better as a senior forum member on car repair forums :) The frig is a Jenn-Air JCD2389DES side by side. The defrost drain is pretty easy to get to. I removed the drain tube + black elbow from the back to clear everything. There doesn't appear to be a check valve or filter/trap of any kind in the drain fitting or the drain tube itself. Please correct if my understanding is correct.
A quick history just for full info. I did replace the defrost control board on this frig a few years ago with the newer design as the old one was not defrosting properly. But that repair probably have nothing to do with a clogged defrost drain.
Thanks for NCARepair for the note. It makes sense, I was thinking similarly last night going over how refrigerator works. The fan on top of the evap coil basically circulates air in the whole frig and there are no filters to trap any dust from entering the evap coil chamber and sticking on to the coil and eventually drip down during the defrost cycles and clogged the drain. This frig is nearly 10 years old so I guess this is probably a maintenance item every 5 year or so :)