The compressor spins freely when the clutches are not engaged, but when they engage thats when it siezes up? Sounds Like you could use a new compressor if thats what your saying.
I don't know how much they cost new, but I'm sure someone on TGO or such has a working one they will let go for cheap, and you can convert to R134a.
OR .............. go the other way and put this part number ( 10055890 )into
www.gmpartsdirect.com