Originally Posted by David C
The part you have labelled "what is this called?" is the viscous coupling.
And 'What's this' is the rear weight which is there to dampen vibration in the prop.
Funny enough a small amount of vibration through the 4wd system is normal on full lock at slow speeds, the theory is that the viscous coupling mounting points themselves are weak and twist under that load. I read somewhere (internet is a big place!) that someone strengthened these mounting points which got rid of all vibration.
The rubber mounts themselves are a problem to get hold of as Stedee points out, i believe one is available as it is the same as a 75 mount (might only be listed as 75 part) but the other isn't available anywhere.
As for steering affecting 4wd, this is only a guess but when the steering is on full lock the front wheels will be moving at different speeds to each other, and at different speeds to the rears, and as the VC operates due to a differential in speed between the 2 sets of wheels it's natural for some force to be applied through the system.
Also, when pulling away the mounts get loaded first before the prop will start to spin which would strain the system too.
At the end of the props are univeral joints, the bushes on which can also wear and cause vibration through the system, so if the VC mounts look fine these would be the next things to look at.
Don't know about normal maintenance, 'if it aint broke don't fix it' seems to work well with P4s, with the exception of cambelts/water pump of course...