I bought a 98 outback to put a motor in, I noticed it still rolls in park...help!

First thing to check is the adjustment of the shift cable and make sure the selector lever on the side of the trans is actually going into park.

 

If that checks out OK you'll have to pull the tail housing to get at the park pawl mechanism.

Broken axle

Broken axle

Or that.

IT can hold park while running, but once you shut it off, it will roll. I had to get by like this, and keep a block of wood in the floorboard to throw under the car as a parking brake. Then when time to go, roll forward and pick it up. Then i chopped a wire to lock in the rear drive and removed the broken axle. then i finally replaced it and restored the wire.  This is a dynamic platform, if you will.