Hi, thanks for posting!
NCrunch has never been integrated with or formally tested with Jenkins, though in theory as long as this tool allows you to specify a console tool to execute, it may be possible to use it with NCrunch. Note that the NCrunch console tool will need to be hosted and executed on the Jenkins server itself (not a build agent) as NCrunch needs to be responsible for facilitating the distributed processing. This means you shouldn't use any of Jenkins distributed build features when working with an NCrunch grid. I'm afraid that Jenkins is outside my knowledge area and this makes it hard for me to further advise on how to set such a system up.
Selenium is a very different tool to NCrunch with a very different focus, and this makes it hard to compare between them. NCrunch is a generic testing tool - it will just call user code via a test framework (i.e. NUnit) and expect this code to perform all logic required for the test. This means that if you are web-testing, you will most certainly need to use an additional toolset to be able to run the web tests inside your NUnit tests.