On a 2001 Chevy S10 ZR2 what locks in the front differential?

The front differential on a 2001 Chevy S10 ZR2 doesn't have a "lock" in the traditional sense like an electronically or manually engaged locking differential. The ZR2 uses a limited-slip differential (LSD).

Instead of locking solid, the LSD uses clutch packs to distribute power more evenly between the front wheels. When one wheel starts to slip, the LSD transfers more torque to the wheel with better traction. It doesn't completely lock the two axles together like a locking differential would. This provides better traction than an open differential but doesn't offer the same level of engagement as a true locking differential.