I've got a TD5 90 (R380 gearbox) and when selecting low range it works for a bit whenever you go to change gear or go over terrain it tends to jump out.

I think it's the linkage but have no idea to what look for or what to adjust? Any idiot guides about?

I usually put her into low range then clutch and try push lever a bit further as mine does the same. Invariably i find it travels the rest of the way the second time. You can also pull forward a meter, stop and clutch and try push it further towards the low range. Hope the helps, it's best case scenario really. I know they can start to seize if you don't use them often.

A good squirt from underneath on all the linkages with WD40 usually makes mine more cooperative, especially if its not been used in a while...