Do the sidekick pinion gears fit right in the samurai diff?
Besides them being from the front axle, you will need some sort of spacer (called "cans") to hold the spider gear pins in place...
Spacers are available from many vendors.
...and yes, you CAN bolt a an aluminum TracKick third directly onto a steel Samurai axle... IF the sidegear splines match the Sammy axle shafts.
Unfortunately, the aluminum housing will shatter like a fortune cookie if you abuse it.