Last month, I had an independent shop replace my 2003 Outback Wagon's front axle and CV joints to unsuccessfully fix a thunk, thunk sound that occurs when the car is warmed up and at slow speed the wheels turned a sharp right or left. Does anyone know if this might be the AWD Duty C Solenoid or ?

Could be.   Try the FWD fuse and see if it changes.  You should get the FWD light on the dash when the fuse is in.  

Search Torque bind and you will find many good posts.

tires the same size and properly inflated?

Thank you Imdew. The independent shop sent the car to nearby Les Schwab Tire Center for a full alignment. The tires were off a bit.

Alignment will impact a bit but different size tires are very bad!

Hopefully they check all of the tires and they match and the pressure is correct.