I'm 17 years old, so I grew up on the old Nicktoons including Rocko's Modern Life. I never really noticed all of the dirty stuff they snuck in so my parents would enjoy the show too.
Just a few examples...
"Rocko and the guys frequented a restaurant called "Chokey Chicken" ( "choking the chicken" is a slang term for masturbation), however the name of the restaurant was later changed to "Chewy Chicken".
Rocko's favorite hobby was "jacking" ( as "jackhammering" is often shortened to). During one episode, Rocko and Heffer walk past a demonstration pit for jackhammers which reads, "Jack all you want." (in that same episode, there was a high-tech, three-headed jackhammer called, "Sir Jack-A-Lot").
Rocko and the guys played a game called "Spank the Monkey", where there was a monkey on the table between them and they all are holding paddles before Mr. Bighead enters the room and asks them to go bowling."
Much more from Wikipedia!