UPDATE: Terry points out that Ford used the marketing term “Sports Utility” in 1966 ads. Previous to that, in 1947, Crosely marketed a “Sports Utility” vehicle.

Crosely (see bottom of the ad):
This 1966 ad for the Ford Bronco has text near the bottom that reads, “Try the Wagon, Roadster, or Sports Utility at your Ford dealer now”. The Sports Utility model is the one with the half cab:

Here’s a 1966 Ford Commercial that identifies each of the models:

Original Post Jan 31, 2019: Dan ‘Scramboleer’ spotted this Motor Trend March 2019letter-to-the-editor about the origin of the term ‘Sports Utility Vehicle’. Essentially, Jeep’s marketing department was attempting to cease the use of the term jeep-like (and similar jargon) to describe offload vehicles. The concern was that it was diluting the value of the Jeep Trademark. So, they came up with the term Sports Utility Vehicle idea. The Journalists took it from there, shortening it to SUV.