will the lockrite locker for suzuki samurai fit in the rear third member of a sidekick?

No they are slightly different. Part number for the Sammi without sidegears is the Lock Right 1510 and the set for the 'Kicks is the 1512 without side gears.
Kelly

I have the samurai lockrite in the back of lil Suzy, It is slightly narrower than the sidekick but other than that exactly the same. I bought the locker without side gears and new Samurai side gears from Brent at TrailTough. The lockers have to be set up properly but I was told that the quality is better on the Samurai than the Sidekick by Brent. He set it up in a new carrier for me, so I don't know the exact install specs but I know it has to do with installing the proper thickness of thrust washers.

Yes it will fit but you need to use the stock cross pins. When I installed one in my hybrid rear the pins kept moving out and rattling on the pinion so I had to pull it apart and install the sidekick cross pin's which use roll pins to retain them.