Encryption allows you to use a public resource (GMRS, for example) for exclusive private use. To have private use of a frequency, you gotta pay.

In the US, you can't run anything but analog voice on the bands you can use somewhat freely (CB, GMRS, FRS etc). On the amateur bands, you can run digital voice but you can't apply encoding for the purpose of obscuring the meaning of a communication.

You can do this on land mobile frequencies, but unless you're an organisation, you won't meet the eligibility requirements to be granted a licence by the FCC.

Your only other lawful option is one of the 900MHz FHSS solutions, though I don't think any of these offer robust encryption.