In order to program a xylophone with an optional tremolo effect you will want to find a xylophone type sound in EXS24, and then create the tremolo effect in the mod matrix.
You could use a square wave LFO, and set that LFO to modulate the volume of EXS24. You would also set MIDI CC 64 (sustain pedal midi data) to turn on/off the effect.
You can use the top orange arrow in the mod matrix to set the “intensity” of the tremolo.
You can set the rate of the LFO via it's speed.
We would choose to use LFO 2, because it is instant on/off, rather than ramping into it (as in LFO 1).