This is the best design only fixed to the rear crossmember and has a ball joint and rubber connector between the door and carrier so no weight on the door. When fitted correctly it also takes away the rear door rattle.
Not cheap though, around £750.
the synchro springs often break they look like a large c shaped spring x 2 ,baulk ring worn ,does it grate ,later baulk rings were coated and this sometimes wears off ,if you can remove rear seal collar and 8 x 10mm bi hex bolts fastening rear...
My experience with terrafirma shocks and springs would say there no where near as good as ome stuff. Springs rust massively quick and shocks snap pins or leak fluid from new. Had 2 sets of terrafirma shocks replaced under warranty before ditching...