funny that, we just bought a new borg warner box last night!
is it the velvet drive model??
stuck a manual on the blog if its any use -
http://bf494.co.uk/assets/borg-warner.pdf
hard to say what the problem is, but sounds like the drive plate is dragging, you check the oil levels in the hydraulics?
if the levels are all ok, maybe the thrust washer and bearing onto the plate?
if you disconnect the shift cable entirely from the box, does its still spin - just to eliminate that its not a problem with the cable, if the cable's a bit worn and rusty inside the sleeve it may bind a little so not return fully to neutral
if the box is goosed we have a tip top borg warner up for grabs shortly 1.5:1 ratio, but pretty straightforward to swap the reduction gear
we only got a new box because we need a bigger ratio