I have a -03 Legacy GT Manual transmission 84K miles w/LSD, that is having some problems with the AWD.
I have noticed that when driving and letting up on the accelerator the car slows really hard. I did not pay that much attention to it. Then a couple of months ago the Viscous coupling went out and caused major torque bind, I had it replaced (not cheap) I am still noticing the hard engine breaking so I did some trouble shooting.
When the car is cold it does not do it but after driving a 1/4 mile it starts and that is a straight away with no turns.
It seems the AWD is always engaged 50/50 so something is causing the VC to lock up. I also noticed when driving on the freeway and put it in neutral there is a vibration and shudder coming from the drive train (feels like unbalanced tires) but if I'm cruising there is no vibration.
I have all new tires, alignment was done, front drive shafts has been replaced, Replaced the oil in the transmission and rear diff (synthetic).

My question is what can cause the VC to lock up?

 

2006 Impreza 2.5i 4EAT, I'm having about the same problem. My problem happens right off the bat though, as I'm backing out of a parking space my front is getting bound and the rear is ok (as far as I can tell). Cruising down the highway I can release the gas and drop 10mph in about 15 seconds or less. I can just was well be on my brakes. I can also hear a slight whine coming from behind me, I can't tell exactly where from though. I don't mean to butt in but your problem and mine might be one in the same. especially the 'wheel out of balance' theory.