propshaft viscous coupling

Can anyone tell me if there is a conversion kit for the viscous coupling on the propshafy

Conversion to what?

Talk to some people they say you can get a conversion kit or a modified vcu unit for the middle of the propshaft

Am I right in thinking the two shafts either side of the unit are constantly at different speeds by design- so the unit is actively trying to iron out the mismatch of revs.
Quite what you can upgrade or change this to- I've not heard of anything? Is it a mechanical diff. Unit of sorts?

Sadly there is no upgrade to the vcu on the FL1. Only new replacement by gkn and recon by reconners.