EMILY: There was something with a big rig. Oh, those things, they scare the life out of me. And apparently, all the men who drive them are hopped up on bennies and goofballs.
Big rig, informal English for a large truck, an 18-wheeler – otherwise known as a semi-trailer truck, a semi-trailer, a semi-truck, or just a semi.
Bennies, slang for the drug Benzedrine, an amphetamine used recreationally since the 1920s.
Goofballs, slang for tranquilisers or sleeping pills, used as a recreational drug. Note that you can’t really get “hopped up” on sleeping pills, and taking them alongside amphetamines seems counterproductive, suggesting that Emily’s knowledge of the drug scene is limited. I think this is her attempt to seem cool and hip in front of Jess.
The Annotated Gilmore Girls 