I have a 1993 ford escort with 100,000 miles on it. When I put it in reverse it shakes pretty good. What could be the cause of this and what do I do to correct it?

Check engine mounts.

Is there a "shake" or is it a thump? A solid thump when changing from park to reverse or neutral to drive, etc, is most likely a failing trans/engine mount(s).

If it actually shakes, like the engine running poorly, you may have a poor ground connection that's causing a misfire condition.