There is no possible way Fez's frog would have been hit by seventeen wheels of a semi-truck.
Kelso wears an orange sash, fake badge, and a name tag made from tape. These would never be part of an officer's uniform, cadet or not. The sash would be a safety hazard.
While Eric is fixing the sink, he mentions that he's handy by saying, "Boba Fett doesn't glue his own jetpack back on". Boba Fett wasn't a character in the original "Star Wars" and was only introduced in "The Empire Strikes Back", which came out in 1980. However, he did appear in the rare "Star Wars Christmas Special" which aired in 1978.
Eric refers to Darth Vader as a "Sith", a term not used in ["link=tt0076759"], the only Star Wars movie released at that point. However, the term "Sith" was used in the novelization of Star Wars, which was published in 1976, so it is likely that Star Wars-obsessed Eric would have known the term.
The lawn mower is clearly a prop. A real mower wouldn't have fallen apart that easily when bumped by the police car.
Red opens the garage door to find a police car. The Toyota & Vista Cruiser are nowhere to be seen, but everyone is home. One or both cars are often missing from garage/driveway shots in other episodes, as well.
The Han Solo figure that Eric shows Red when testing him at the kitchen table is from the 1990s "Power of the Force" figure line and not from the '70s.