i have a customer with a 2000 hzj79 pickup that has the r151 and a split case in it, all oem from toyota. not sure where you're going to find the proper extension housing, but it'll most-likely have to come from canada or oz.
i know proffitt's has a few hzj trucks, might be worth asking them.
The trans in question is quite possibly designated an R151 but not the same one most associate with the'86-'87 Turbo Mini Trucks. Most would refer to your trans as an R150F.
In reality, Toyota uses a number designation to cover only a span of 10 years. The R151 designation for the '80s was for the Turbo truck trans. For the '90s, the R151 designation was put onto some of the trans' most refer to as the 3.4 R150 trans.