1995 gmc k1500

Check the trans fluid level. You may also be experiencing a restriction in trans fluid which may be affecting the torque converter (dirty or loose filter, or restricted trans cooler for example). This in turn causes a hard shift which is enough to bog your engine down. (reverse just happens to be the earliest sign of this problem as it requires the highest level of trans line fluid pressure, eventually it will start to stall in other gears too). Then you have to address the stalling engine, which can have any number of causes - dirty throttle body, low fuel pressure, needs a tune up, restricted cat., etc. The AC is likely only adding an extra load to the engine when it is borderline to stalling.