That part is not hard to install. If you were able to get your chain back on you should be able to do it easily. The part number is 41C4220A. I no longer remember the price. One thing you need to check is the condition of your door. It is usually a problem with the door that causes the bearing to go bad in that assembly. It is called a 'gear and sprocket assembly'.

I have changed this part many times. If you are able to make the change, then is likely worth it,(if you are going to pay Sears to do it, then don't do it. You will pay more than it is worth) You can get them reasonable on ebay. It is the top metal gear, and has a large white gear connected on the bottom. I don't recall the part number, as I recycle them from removed units,(I beleive the part number is listed in the post above) It is mechanical surgery, if your okay about that. Remove the door opener power head from the ceiling and put it on a table to do this one. You may want to take a picture of the internals before you start removing things to get at the part to change. Dan