First a couple of Transfer Case notes. You will frequently hear people talking about "3-speed" cases and "4-speed" cases...this has nothing to do with the number of gears in the transfer case but rather what type of transmission the case was installed behind. All Cruiser transfer cases have your standard high and low range.
As can be seen in the cases below, Toyota made a number of changes to strengthen them over the years. The '64-'66 and '67-'69 cases are really the only ones that need to be avoided. These cases are made of brittle aluminum and more prone to cracking. Additionally, the idler pin support has little material or ribbing around it. The '70-1/75 case is really the first desirable case for offroad use. The idler received a heavier wall around it and additional gussets were added. [click the photo below to better see the changes]
Additionally, the easiest tell-tale sign between the brittle and soft aluminum cases is the boss around the drain plug. The graphic below show this very well.