I needed to do the same thing, he's what I did.
I got a 1/8" male stereo to 2 1/4" female mono cable, it splits the signal into left and right. Then I used the 1/8" female to 1/4" male adapter that came with my headphones and plugged it into either the left or right female plug of the cable depending on what I wanted to hear.
It worked great for transcribing panned solos (Trane stuff mainly) and playing along to some, well, play-alongs. haha
I hope this was clear enough to get, it's a shame Apple doesn't just add a balance feature on the iPods.
Here is a link the cable I got.
http://www.bhphotovideo.com/c/produc..._1_8_Male.html
Good luck!
~B