I have got a gas strut on mine https://www.ebay.co.uk/itm/Gas-Strut-Rear-Door-Stay-Kit-Land-Rover-Defender-to-2002-BRITART-DA1204-/371041055675
Fits on a series ok...D
I had a similar problem but a bit more random, finally replaced the clutch, turned out the splines were rusty so the driven plate was not fully backing off the flywheel
http://web.archive.org/web/20170223121919/http://forum.landrovernet.com:80/misc.php?do=vsarules&doredir=1&cfrset=1
There are website archives but would not know how to access all of it, the link above is from feb 2017