Your AC compressor may be bad and when you turn the AC on its putting too much stress on the engine and its trying to overcome it. My aunts trailblazer had ceased bearings on the compressor and when you would turn the AC on the engine would drop to 500rpm and then die.
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Exploration Machine- 02' CC V8 swap. 300K+
Portable Roller Coaster- 07' MKV VW GTI
Its another Jeep- 98 TJ