1999 Mitsubishi Galant Crankshaft pulley replacement

About 2 months ago the outside belt slipped off of the engine pulley. I replaced it. Then about 2 weeks later a noise developed under the hood. It was not a constant knock at that time. It only knocked when I turnde the wheel. I opened the hood at noticed that it appeared that the crankshaft was oscillating towards the front wheel well by 1/4 to 1/2 inch. A mechanic told me that the crankshaft engine bearnings had probably gone bad and the crankshaft was moving out. I drove the car home and the knocking got worse. However, the car is running fine. I did some online research and feel that it could just be the crankshaft pulley itself. If it were the crankshaft shifitng wouldn't the pistons and valves be slamming around? I would expect oil to be all over the place? I am a little mechanically inclined. Is it very involved to replace the crankshaft pulley?