4wd engaging on its own whilst driving

If the transmission has wound up due to driving on a grippy surface it should just refuse to come out of 4wd until you back up to release the load. It shouldn't pop back into 4wd.

Put it in neutral then jack up one wheel to make absolutely sure you have no load on the transmission and check if it switches between 2wd and 4wd correctly. It does not matter which wheel. The lever should have a positive click is it switches between modes, similar to when you change gear but a bit firmer.
If it's sloppy with no real feel for when it changes between 2wd and 4wd the detent could be faulty. It's pretty rare though. The detent is literally just a ball and a spring so there isn't much to go wrong.
If it is very stiff to move it could be simply that it has seized up due to lack of use. In that case a bit of oil on the linkages and a bit of exercise should sort it.

Not an expert on this but what about a vacuum pipe issue. We had a problem a few years ago when we had sudden juddering but can’t remember the details fully. Ours was something like grease in a newly replaced vacuum system

Paul.

Only the hubs use vacuum. Issues with that system can leave you with 2wd even in 4wd mode but it can't cause the transfer to jump into 4wd.

Its feels to be clicking in to place really well. Ill get it jacked up over the weekend and check. Had a drive about on the wet grass earlier and it was deffinetly changing between 2wd and 4wd properly. 

If it worked correctly on grass I'd say it's all working fine. I suspect that your original issue was transmission wind up due to testing on a grippy surface. When you thought you were switching out of 4wd it wasn't actually able to switch due to the wind up. I wouldn't worry about it unless you have problems again.

Ok thank you for the help once ive sorted out my brakes ill take it for another drive and update if i have anymore issues with it