2000 Chevy Blazer Connecting rod bearings

I just bought this blazer less than a month ago from a private owner. It had a slight knocking noise that proceeded to get worse upon doing an oil change and tune up on it. I brought it to a friend of mine who suggested it might be the connecting rod bearings, as the knocking worsened while accelerating. Today, I brought it to a garage and they told me it is the connecting rod bearings and the only way to fix the problem is to replace the engine. Is this true? Is there no way to repair this problem? I have no recourse for this with the previous owner unless I can prove he knew of this problem and didn't disclose it due to the lemon laws regarding private sales. This was my son's 1st truck and we are out $2,000 that we paid for it, which is also what it will cost us to remove the motor and have a new one installed. Any advice? Also, how long could this problem have existed before breaking down? This couldn't have just happened in the last 3 wks, could it?