Like derass said you could swap the ring and pinion for a lower gear, but I would definitely have a shop do the work.
For all swap info check out
http://www.ka24development.com/vlsd.html
The s13 diff won’t bolt into the J30 easily. The J30 diff is slightly longer than the s13 open diff. It has a speed sensor on the nose that makes it about 3/4”longer. It also has a different front bushing setup with longer bolts and washers. You need the bolts and washers for the swap as the factory s13 ones are to short.