Hi,
I'm totally new to laning as well and I'm after finding some easy lanes as well- I've never been. I think the best place to find out where they are is OS Explore but if you're new to it (as I am), its also best to ring up the local authority to check that the lanes are still open for public use, as sometimes they get closed for various reasons, and if you get caught on them the consequences are dire!
I walked a little way down the one between Crowland and Peakirk the other day, but it is just a gravel track. I'd be up for checking it out, as I'm more in it for the the countryside than 'pushing my vehicle to its limits'.
I just have a standard x-trail with all terrain tyres. I think you'd be surprised what your SJ will do. I saw some totally standard at KORC and they were keeping up with the best of them!
Let me know if you are interested in meeting up and figuring out an easy route! where abouts do you live?
Reubs