You are probably referring to the LLV, or Long Life Vehicle. Commissioned in the earlier 1980's, introduced in 1986, it was designed by Grumman ( noted for their rugged airplanes) and somewhere over 110,000 of them were built and went into service. As the name implies, they were supposed to last a long time, and they have, typically running for 20-30 years. Easy to repair, with a non-rusting aluminum body ( yes, like an airplane, there) rugged suspension for handling all sorts of bad roads, and a tight-turning radius, they are one of those little technology success stories that most people don't notice.

They do have problems in deep snow, and of course 1987 was before hybrid or electric vehicles were commonly thought practical. They are going to be replaced soon, it looks like, with something more energy efficient.

The Smithsonian has a Postal Museum, here's the blog on the LLV.