Locating Set Of Points Close To One Another (within A Threshold)

Hi all

I have a large data set of points situated in 3d space. I have a simple
primary key and an x, y and z value.

What I would like is an efficient method for finding the group of
points within a threshold.

So far I have tested the following however it is very slow.

---------------
select *
from locations a full outer join locations b
on a.ID < b.ID and a.X-b.X<2 and a.Y-b.Y<2 and a.Z-b.Z<2
where a.ID is not null and b.ID is not null
---------------

If anyone knows of a more efficient method to arrive at this results it
would be most appreciated.

Thanks in advance
Bevan