Probably CV joint(s), or front differential. Remember on an S-10, the
front axles and differential spin all the time, regardless of 2or4wd.
Only the front propshaft stops in 2w. (That's part of how shift on fly
works, the connect/disconnect occurs in the front differential)
If you can, lift/jack both front wheels off ground, manually turn
wheels, source of clicking should be fairly obvious.
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