Looking through the rules on Knockdown, I can only find reference to using melee to accomplish such. Is it outrageous to assume I could fire a shotgun slug into some drekheads knee in order to knock him down? I figure if it comes down to it I'll just adapt the called shot rules, but first I wanted to see if anyone here had come across a solution to this yet?

Knockdown is built into every attack. The melee option just increases your chances for success by forgoing to option to do damage. The closest you'll come to using a ranged attack to force someone prone would be suppressive fire, which is completely different from knocking someone's kneecap with a 12 gauge rifled slug.

^^
But doesn't it make sense to have an option for knockdown via gunfire? I dig that I can knockdown anyone potentially, by causing enough damage, but IMO getting shot in the leg with anything beyond some small caliber firearms, should require somekind of test by the defender to remain standing.

That's one of those "Up to the GM" situations. If the player calls a shot to a leg, (assuming you're allowing such as a GM, because SR4 mechanics don't specifically allow location specific shots to things like knees... simply to "vulnerable locations"), then the GM can call for a knockdown test or what have you.

Hell, even a 9mm will pretty much destroy a knee, so a person may need to make a test just to stay standing due to lack of support from that leg.

Alternatively, a called shot for bonus damage or negating armour, plus an aim action to offset some of the penalty, plus a good roll giving you nice damage boost. Then your GM says "well, looks like you kneecapped the guy, he isn't getting up" and we move on with our game, and lives.

Ultradetailed mechanics aren't necessary, and only get in the way of play because you end up digging through the books looking for the hit location tables.

Seems like a clear cut case of "...other specific effects for called shots..." The rules seem pretty clear.

If you look at page 151 BBB the rules for knockdown state:

Anyone who who takes a number of bopxes of damage (Stun or Physical) from a single attack that equal or exceed his Body, then the attack automatically knocks him down. Anyone who takes 10 or more boxes of damage in a single attack are always knocked down.

*Note Certain less-than lethal weapons are designed to knock a target down, including gel rounds and shock weapons such as tasers and stun batons. Gel rounds reduce the Body of a charachter by 2 when comparing it to the DV to determine knockdown. Shock weapons have their own effects, noted under electricity.

As far as targeting a specific body part to knock someone down the last notation under called shots states: The gamemaster may allow other specific effects for called shots if he chooses.

As a game master I would allow PC's to knock down an opponent with a called shot to the knee or leg and use a dice pool modifier appropriate for the situation. If the target was running for example it might be a bit more dificult to hit just the right spot but fairly easy to trip him up. If the target was standing facing you only a few feet away I think you could pop him right where it counts without much dificulty at all. So applying a modifier of -1 to -4 like the other called shots would be appropriate.