I know that there is an adapter available to mate the two cases together for a wide selection of gears, but is it possible to simply replace the sidekick case with the samurai's?
I don't know that
simply is the correct word to use.
Yes, it can be done. Probably the best way would be to use a 2wd tracker tranny. Otherwise, when you remove the tracker tcase from the tranny, there won't be any support for the tranny output shaft. Then frame mounts will have to be made to bolt the samurai tcase to (the samurai tcase is a divorced, where the tracker tcase is a married design). Then a short shaft that runs from the tracker output shaft to the samurai input shaft will have to be made.
Stock tcase ratios:
tracker high 1:1, low 1.82:1
samurai high 1.409:1, low 2.268:1
So yes, swapping to a samurai tcase will have a noticeable affect on your high range. If you are running 5.12 diff gears, it will make them feel like 7.22s, if you are running 4.62 diff gears, it will make them feel like 6.52s (if I did the math right).
What gears are you running in your diffs now? If you don't know, do you have an auto tranny or a manual tranny?