I do believe that the difference is as follows:
Reductio ad Absurdum is a valid argument in which you prove or
disprove something by following logical conclusions until you find
an absurd statement.
Ex.
Father: Why did you start smoking?
Daughter: All my friends were doing it.
Father: You're saying that if all your friends jumped off a
cliff, you would do that too?
The father uses reductio ad absurdum by following the logical
conclusion that the daughter will do anything her friends do, the
result of this is of course absurd.
Reductio ad ridiculum however, is NOT a valid for of argument
and just makes you sound uneducated if you use it. It is taking a
statement to an extreme, however correct level as an attempt to
discredit it, this however is unfair and does not actually
challenge the argument.
EX
Borrowed from Wikipedia
If Einsteins theory of relativity were true then when I drive my
car it would get more massive, now come on that's ridiculous! (This
of course is true, although humans can't detect it)