i have a three door xr4i on a b reg. What lsd will fit straight on, without changing drive shafts hubs etc. do i have to open the diff up and change housing or does everything just bolt and plug straight in? does a saphire one fit etc, whats the easiest way, is it worth welding the diff or will that cause the car to many problems, what needs to be welded? i know nothing about rwd fords, i have always had rs turbos and xrs, i have a saphire cosworth now and im always a bit scared to power it out just cause its cost me so much bl**dy money, so i bought the xr4i cause it was going to be scrapped but the shell is solid, unfortunatley there is some dents and scratches and it does need a servo, but i thought it would be a perfect oppurtunity to have an every day sideways action Three door but with just one wheel spinning shes far to snatchy for smooth sideways driving like the cossie. please help me.

 

Either of the diffs will fit from xr4x4 or from cossie.
Some are 7" wide some are 7 1/2" wide, can't remember which is which!! for the narrow ones to fit wider beam just put a few washers either side of top bolt hole, between diff and side support of beam, for the wider ones in a narrow beam just cut off 1/4" either side of the top rear mounting bolt.
tabetha

 

Im pretty sure that the rear diff you have is a 7inch.
the most straight forward option for you is to put a 4x4 cosworth rear diff in as they are the same size
where as the 2wd had the 7 1/2 inch,

 

when i said "2wd" i meant that the 2wd cosworth
they had the 7 1/2 rear diff
as normal sierras only normally have the 7inch that i'm aware of