The Syncro is the same system as the Rallye, G60 and Ltd.
IIRC the higher power Syncros have a higher-torque viscous coupling (VC) at the back. The rear diff is pretty much the same, though and all are independently lubricated.
The main difference Rallye to Syncro transmission-wise is that the Syncro has a rod-change box, the design of which is much weaker - the Rallye is cable-change, which is a lot more robust. The two boxes DO have a transfer box to take power to the back, but it's not known as particularly weak. There are a couple of differently lubricated (independent/non-independently lubed) transfer boxes that were fitted to the Rallye.
The Syncro-system (as mentioned above) relies on a VC. This is a torque sensing device, mounted in front of the rear diff, which has viscous fluid in it. The action of the fluid is similar to a cornflour-water mix, if you stir it slowly it's liquid, if you try to stir it fast then it goes solid. This device means that the 4WD is controlled entirely by driving conditions, with no input from the driver. It is better described as 'pseudo-full-time' rather than 'part-time'. This is exactly the same as the G60, Ltd and Rallye. Heat can improve the response time of the unit, but it works from cold.
The driving experience of the Syncro is hugely different to a GTI or other 2WD Golf. Like the Rallye, Ltd or G60 Syncro the carb Syncro shares independent rear suspension and also has the stiffer high transmission tunnel bodyshell identical to the G60 Syncro and similar to the (massively reinforced and very heavy) Rallye shell. With a Rallye ARB on the back to dial out the standard 'buckets of understeer' handling and a few other tweaks the Golf Syncro will run rings around many other cars, especially on back roads, or in the wet. A well tweaked Syncro would be able to keep up with a Rallye, too - it's a LOT lighter.
If you kept the original gearbox, fitted 280mm brakes on the front and the 16V master cylinder, maybe some stiffer springs and the Rallye rear ARB, swapped out the GU head for a PB head with a nice lairy cam and fitted a Weber 32/34DTML carb you'd have yourself a cheap, light sleeper that would be a lot of fun to drive, too.
That's what I have, that's how I know how good it is.
Sure as hell beats trying to source obsolete parts for a lardy Rallye.