i have a 91 4x4 4.3L S10 Blazer and when i am in reverse or making sharp turns there is this sound coming from my rear left tire thats kinda like a clunk noise, does anybody know what this could be

check your suspension bushings. they are probably rottted or damaged to a point that they are cracked or missing. how does the vehicle drive? does it pull when you brake? does it wander when holding the steering wheel straight? do u hear clunking noises over bumps? do u have a wear pattern on the tires?

Yip, or your rear drum brakes are falling apart.

Jack each side of the rear of your vehicle up and check side to side wobble and in and out play. You are checking for a bad rear axle bearing.

the only one it does out of those is the clunking noises over bumps, but thanks guys this helps a lot