The least expensive locker for the 10.5" Sterling is the Powertrax Lock Right. The Lock Right for my 10.25" Sterling was about $300. I had it installed at the same time as the gear change.
A gear change with a Lock Right install should cost about $1000 for your 10.5", depending on local labor rates.
The Lock Right fits in the stock open carrier, which is why it costs alot less than the comparable Detroit Locker. Both the Detroit and the Lock Right are all-gear ratcheting automatic lockers. There are no clutches to wear out and no electric or air system to install to active the locking mechanism.
I've been quite happy with the Lock Right in my F350 for the last 12 years.
01 International 4800 4x4 CrewCab DT466E Allison MD3060
69Bronco 86Samurai 85ATC250R 89CR500
98Ranger 96Tacoma
20'
BigTex flatbed
8' truck camper, 14' Aristocrat TT
73 Kona 17' ski boat & Mercury 1150TB
92F350 CrewCab 4x4 351/C6 285 BFG AT 4.56 &
LockRite rear