If it is only on acceleration,
I would put my money on a U-joint some where in the prop shaft (driveshaft)
I just replaced mine. There is a big heat shield covering the front two joints, so you may not be able to see it.
You can sort of check it by putting the 10amp fuse in (under the hood center, driver side) in the 2wd spot. This will disengage the rear wheel drive.
http://www.subaruparts.com/diag/?mod...category=199-1