I just replied to your pm but will say that you should have enough info to decide if you want to buy this $400 diff or just wait until you can get the right one.
The extra strain on the gearbox might not be that bad if there is only a 20% difference in speed and you take it easy.
I prefer to do stuff right the first time as I'm too lazy to do it twice but I'm sure you can decide what to do.
Gtg I have homebrew that needs attending.
You gotta do some maths to see how the new diff will affect your speed. The btr already has a crappy first gear ratio so I wouldn't want to make it worse.
Perhaps a visit to the local diff shop may provide a reasonable price to repair your car as is.
There's no such thing as a cheap fix. You will end up paying what you should have in the future on top of what you pay now.
Ebay is full of parts.
http://www.ebay.com.au/itm/BA-BF-XR6-UTE-DIFF-/272558837054?hash=item3f75c5cd3e:g:8fUAAOSw4CFYpZJw
http://www.ebay.com.au/itm/Ba-Ute-Diff/282037512571?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D20140106155344%26meid%3D141a2aaa0db14b6bacfb7d3500e8409a%26pid%3D100005%26rk%3D3%26rkt%3D6%26sd%3D272558837054
http://www.ebay.com.au/itm/FORD-FALCON-BF-BA-UTE-DIFF/112035959458?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D20140106155344%26meid%3D79ad6e5e1f65473b8312a06201550ca3%26pid%3D100005%26rk%3D6%26rkt%3D6%26sd%3D272558837054
etc etc.
My kids leave the back door open regularly and the chickens come in, eat the cat food and sh*t in the floor.
I trod in poo last night.
Besides prickles, that is why I wear thongs everywhere.