Back up sensor not working properly

This is an fj crusier, when I put it in reverse, inside the car beeper lets me know that it's in reverse, but as I am backing up, and getting closer to anything, I don't hear anything. This problem just happen the past few days and was working. I did check the connections I the rear and they are plug in, the fuse is good. Could it be "both" are bad, or if one doesn't work, then both don't? They seem very hard to replace, which are under the rear bumper and looking at you tube videos, it looks like a pain. Any help on this issue? Thanks