2007 Taco AC issue

I have a 2007 Tacoma access cab prerunner with a 6 cylinder engine. About a year ago I noticed the AC was not putting out as much cold air and/or cooling the cab down near as fast as it had in the past. I have a mechanic at work that checked the freon and the compressor. Both are fine. I replaced the cabin filter and it was quite clogged but still same problem. The mechanic told me that the evaporator coil may possibly be clogged with dirt and debri from years of off-roading. He suggested going to a auto parts store and buying a cleaner that could be sprayed up into the drip hose. He said this stuff would foam up and eat away any grime and residue that may be there. I went to a couple of places and they had no idea what I was talking about. He called several that he deals with and was told the product is no longer offered. So I'm still having problems. I notice that the air coming out of the drivers side vent is quite cool, the drivers side center vent is less cool and gets even less cool as you go towards the passenger side. Seems to be plenty of actual air pressure across all 4 vents. I removed the cabin filter to see if that would help and no change. I also notice that when I turn the AC from off to low I hear a slight whistling sound, it gets a little louder as I go higher in the AC. Any help would be appreciated.

 

I feel so alone......