Anyone know if a 91 or newer tcase will fit in a 90 sidekick jx? I want to put low range off-roads gear reduction kit in it.. but it only works in a 91 or newer... something about my housing being to thin.

It will fit but if you get newer than a 94 you'll have to change the yoke on your driveshaft to fit the different spline count on the output shaft of the newer cases. I have a tcase that will fit your truck with 4 to 1 low range gears installed and twin stick shifters for sale. It already has the correct output spline count for your current drive shaft if interested.