Y61 Patrol ZD30 manual drops out of first gear

I have a 2001 GU ZD30. The engine was recently rebuilt. Recently it's developed a malfunction in first gear in that the gear stick has become slightly sloppy and after putting it in first it simply drops out again. If I hold it in while I take off it's quite happy to stay there under load, and doesn't always drop out on over-run. It happens whether in two or four wheel drive, low and high ratio. It also feels a bit loose when moving to second, but it doesn't drop out of second. I think it's probably a problem with the detent spring, but I'm not familiar with Nissan transmissions. Has anyone had similar issues or know what might be amiss, and how to fix it?

Thanks,
Rayk