thanx for the help guys, but i managed to do it by myself.
and i devised a simple way to mirroring any animation
the idea is not to mirror an already animated part. it is to do both animations at once
here's how i do it:
to start, i animate the first keyframe of the left part normally.
to match the translation, i align the second part to it. (up/down, fore/aft axis only)
then i open the rotate type-in thingy (right click on the rotate button on the toolbar) and select the left part. i take the absolute rotation values and copy them to the right part. (i have to add a minus (-) in front of some of the values so the rotation is mirrored, not copied.)
now the right part is in the EXACT same position as the left one. i just keep repeating this procedure for all the keyframes
remember that for this to work, both parts must have their pivot points centered, or at least equal to the other.
it's not as simple as just clicking the mirror button but i think it's the next best thing

c ya
Moach