I think there should be an option for a welded diff. You can choose to have drift steering and it just doesn't make sense to me why there would be no welded diff for a drift build. Also, i noticed the rally config doesn't have a skidplate. I would expect a rally car would have a skidplate. I seem to remember one of the newer cars didn't have a skidplate when it came out, but it was added a week or so later, so im not really worried about it. I think the devs willl add a skidplate, but im not sure if they will add a welded diff.
To make a diff that is good for drifting, just put in the Race LSD, then in the tuning tab move the LSD torque all the way to the right.