Which Mork and Mindy episode contained the segment where
Mork and Mearth were sitting in the Jeep and pretending to be in a
car race, and how can I get a copy of that episode?

It was from the fourth season, but I don't know which episode it was in. The fourth season isn't available on DVD yet.

"Drive, She Said" is the title of the episode, Mindy is tired of driving Mork everywhere, so she tries to teach him how to drive. They were in the parked jeep when she finally had enough and got out. Mork and Mearth then proceeded to pretend they were in a race.