1999 Subaru Impreza Outback - Shake n' Shudder

Are all 4 tires precisely matched as to brand, model, and size?
Do all 4 tires have the same amount of tread wear (or does it vary from tire to tire)?

If the answer to either or both of these question is “no”, then it is very likely that you damaged the clutch pack for the center differential.