External effect

What do you have the TC hooked up to interface wise?  It should be as simple as SONAR sending out a signal to the interface and the interface passing the signal on (and returning it) via s/pdif.

@

X2 will not recognize the unit - it will "see" the input/output digital of the tascam that you have hooked up to the tascam.  X2 has no way to know is hooked up, except for midi.  So be sure and hook up the midi.

Then it is only a matter of sending the track you want to effect to the tascam digital out and then routing the digital return to SONAR's master (or a new track if you just want to record IT).

Signal flow can get confusing when you are dealing w/ a DAW, interface and external equipment.  It wasn't designed for it, unlike an analog board.
@

Sync the clocks......I guess you know what the next question would be. Thanks Middleman

It might do it automaticly. If your getting sound then all is good, otherwise you'll get a wordclock warning either from Sonar or the TC. 
Best if  Tascam ( Sonar)  is the Master. It will also depend on if the TC supports the same clock rate you've been using to record in Sonar. I have to use 44.1 as my Yamaha 01v does not support 48 or higher. 
I have a us1641 which is the same and I have used external devices and the SPDIF all the time. 

Here's one way to do it, there are others. 
Insert a new audio track and point it's input to the SPDIF which will show at the very bottom of the input list. Use stereo.   

Now set up a bus that is pointed at the SPDIF output. 
Buy adding this buss send to any track you'll be able to route the signal through the effect. The effect will be printed to the track you made. From there you can do a few things for mixdown. I assume your after harmonies. 

Don't forget, you can only use the SPDIF output OR  output 3/4. Cannot use both at once.