I've not dissected a Passat set up so I'm not 100% on the detail differences. I know it uses different (longer) driveshafts. I also can't remember whether the 2WD 35i Passat used a torsion rear beam, but if it did then I suspect the underbody story there would be much like the Golf one - big differences.
The big difference Golf Syncro vs the higher Golfs and the Passat is the cable-change gearbox - a much stronger unit. There are myriad differences though. The rear diff unit on the Syncro set-ups from that era only has detail differences between the models. It's all very similar in that area. Except the VR6 models, which use a different (Matched) final-drive ratios, as well as a different bellhousing for the 6 cylinder motor.
On a 2WD UK spec Golf you're into replacing the tunnel with the high-tunnel out of the Syncro, and the boot floor, too. Not hard..... if you're good at cutting and welding. It'd be a b!tch if things weren't straight though!
The high tunnel is a massive benefit to the shell as it stiffens the whole structure massively - you can jack up one corner and the shells just don't sag at all. Obviously this has benefits for the handling of the car, as well as the independent set up in the back, which stops the Syncro cars cocking a wheel in corners.
The only Golfs that got the high-tunnel shell in the UK were the Golf Syncro. In Germany it was used on the 2WD G60 GTI as well as the G60 Syncro and the Limited, of course.
The Rallye shell is much heavier than the high-tunnel shell. It was designed to support VW's drive into rallying, so has masses of reinforcement, which really isn't necessary in a road car. Oh and it's got two less doors and most of the panels are unique to it. So aside from that, yeah it's pretty much the same shell.....Both models were fitted with bonded screens though.
The high-tunnel shell is the best compromise in my eyes; it gets you a car that looks like yer regular Golf, rather than a Rallye, which sticks out like a sore thumb full of obsolete parts.
