Check the audio forum here - it's very helpful. Search on XM or Scion HU and you'll find more than you need. There's even a very nice writeup on removing the dash panels to replace the headunit.
Best solution (cause it's the one I picked) is to get a Scion headunit, which is a direct replacement for the non-jbl unit, and a Toyota XM reciever.
The Scion HU looks factory in the 4Runner and sounds a lot better than the stock while retaining steering wheel controls, rear-seat audio, etc. You'll get XM channel number, be able to change channels, see artist info, etc right on the display (with the XM reciever, that is). They can be found on eBay.