Sometimes the 17mm tranny plugs are darn tight.  

You can weld two nuts together (in the center) to make a tool or you
may want to buy a special socket like this:
http://www2.cip1.com/ProductDetails.asp?ProductCode=ACC%2DC10%2D7165

Also, if you are draining the old fluid, you can try switching to
Redline MT-90.  Better than regular gear oil.