I have an English axle with a 3,6 or there abouts, can I fit a 4.1 cwp and keep the diff unit as I've a tran x diff in there? Is it just a swap or having all the diff setup again

Thans

My understanding is that you will need to have it set up again.
It's to do with the back-lash between the crown-wheel and pinion each one having a diffeent amount of back-lash.
I'm sure if you google/youtube it you'll find a more specific answer.
HTH.
Cheeers, Pewe10

You need to set it up again. As above the backlash needs to be set, but to change ratio you need to change pinion so the pnion preload will also need set.

Cheers
Davie

Time for a rebuild?! That is what I did with mine - switched to a LSD, and had a number of seals/bearings replaced before having it all setup again. If you do have it done - talk to ProComp, and ditch the crush tube (the cause of many a leaky axle) for a solid tube instead.