are you saying any unit of that type drops the item? if so do this...
condition-unit type of "dying unit" = "your unit type that you want to drop item"
action-create item at position of dying unit ^.^
Set X to Random number between 1 and 10
(.IF THEN ELSE stuff,)
If X = less or equal 3
THEN create Item1 at unit position
IF X = greater than 3 AND less or equal 7
THEN create Item2 at unit position
and so on
Here we have a 30% chance on Item1 and a
40% chance to get Item2, if u let it like this we
do have even a 30% chance to get nothing because
of the 8,9 and 10 that are not specified here.
At the conditions, u can use a Unit type or a Specific Location
whatever your imagination allowes.
Try to not use 'Waits' because this is not a local Trigger.
If i would be at home i would do this for u, but sry.
The problem is that if u use 'dying Unit' and between the
Start and the End another unit dies then there would be
2 items at the new dead unit and no item at the first died one.
But without Waits i dont think it would cause problems.