What are the common signs that indicate the InSinkErator Xtr 3/4 hp needs to be replaced rather than repaired?

The number one telltale sign of a garbage disposal about to give out is sluggish operation. If the disposal sounds like it's strainning and you can actually see it doesn't grind well, it's time to look for a replacement. Note that this is recommended only when the warranty has expired, as may miss out on free repairs or replacement. Make sure you read and understand the warranty claims that comes with the disposal at purchase.