Mine has the same symptoms. From what I've read here, it may be caused by the use of aftermarket axles. I had mine done at the dealership when I first got the car, so I assumed they were genuine Subaru OEM parts, and still get that shudder in drive that goes away in neutral.
Stuff that I have done in the meantime (for other reasons) that have not changed it: New plugs, wires, coil, alternator, timing belt, pulleys, tensioner, water pump, brakes, and struts. So, you can check those off the list. I plan on changing the AT fluid and filter when it warms up a bit and will let you know if that helps. I don't think it's serious - but will be watching this thread for others' opinions.
