I don't think a locking rear requires anything different. Limited slip rear should use gear oil with a LS designation. I recently did my 4runner (front, rear and transfer case) and it was not that hard of a job. Bought synthetic gear oil cheap online. If I had to guess I would say a dealer would charge around 350, maybe more. Call your dealer first then start asking around.
Thanks to all for the info. Paid $354 for the front & rear differential and transfer case fluid changes. 75w90 synthetic in the differentials and Auto Trak II in the transfer case.
Missed it by 4 bucks.
You can do it yourself next time for under 100 bucks easy with new washers. BTW, washers should never be reused.