On lock rites, (I run front & rear in the Tracker) are just entertaining in snow & Ice. the only time that I've found that they may be dangerous in is in the intermitant stuff. ie.. part dry part ice.
As far as the rear wheel drive car w/ a posi or good limited slip. it was your tires that kept moving your back end down hill. with a light foot and good tires a traction system is awsome. (ex-cop, lotts of experiance with big powerful rear wheel drive cars)
When it comes to driving with the auto lockers, such as Lock-rites, is that they lock and unlock. this makes it hard to drive for your car as it is always changing. it can be done and done well, but takes practice and an easy foot (like that's hard in zuk).