So, there are still a couple of unanswered questions:

-can the input shaft be swapped between the auto and manual T-cases?
-will the front/rear driveshafts need modifications?

In my mind, converting an automatic Previa to an earlier T-case would be viable for the following reasons:

-increase fuel economy by using 2wd most of the time, and 4wd - on demand,
-convert, because the original viscous coupling in the Alltrac T-case went bad.

Installing a 2-speed T-case is a bonus, but for the automatic Previa really isn't necessary, as the auto tranny would take up the slack in most situations where a low range would be desired. Combine that fact with the ease of conversion, and a push-button control for the 4wd with the auto T-case (single speed), and I'd be tempted to install a single speed T-case instead of the 2-speed T-case.

Converting a manual Previa to a 2-speed T-case would make more sence though.
But it's not like Previas are really off-road capable, with the long-wheel-base chassis. More like snow and sand kind of off-roading, and a Hi 4wd will do just fine.

Just my 0.2c worth...

Val