I have a 1991 s10 4x4 blazer and whenever i turn on the A/C it starts to vibrate and shake a lot. Any idea is to what the problem is.

Compressor could be low on oil, compressor shaft bearings could be worn, the clutch hub could be faulty, compressor itself could have loose mounting bolts, could even be something as simple as the system is overcharged with refrigerant.