What is the difference in Israel between "sherut" and "sherutim"?

Sherut is the Hebrew word for Service and it's plural is sherutim. It has other meanings, where sherut means 'shared taxi service' and sherutim means a WC!

http://www.jewish-languages.org/jewish-english-lexicon/words/511

https://www.haaretz.com/israel-news/which-way-to-the-sherutim-podcast-1.5377313

Sherut is the Hebrew word for service in general. Sherutim os a bit more specific - services (plural) provided by some organized body or law. The word "sherutim", when used, generally refers to "the facilities" (the bathroom), but can also be used to refer to things as grand as the criminal justice system (as "the services of the law", "sherutei hachok"). I'm currently preparing a few notes to send to a writer on the subject of Hebrew untranslatables, in which I'll elaborate on the subject further.

"ha-sherut" ("ha" means "The") is the common name for the Israeli GSS (General Security Service, literally translated from "Sherut haBitakhon haKlali" hence also commonly abbreviated as SHaBaK).