Their mortarboard tassels show the grad year of 1978. The banner at the group's 10-year reunion (s4e1, "It's A Wonderful Life") says "Welcome Class of 1979 10-Year Reunion".
Kelso says "that one guitar-shaped log is roaring" then realizes it is his guitar and a fight breaks out between him and Hyde. During the fight scene you see the guitar on the fire but not really on fire. Then a bit later in the fight scene it is completely gone from the fire.
In the scene where they are standing around the campfire and Kelso claims that there is a 'guitar shaped log' in the fire, we can see the guitar. However, in the next shot, the guitar has disappeared.
Near the end of the episode, Red hands Eric a ten-dollar bill for gas. If you look closely, you see that the bill is the late '90s version in a show that takes place in the '70s.
Eric mentions the mountain air just before they find out the van is missing. There are no mountains in Wisconsin. Most of the state is quite flat.