Do you mean the physical transmission case is broken? Are the internal gears, bearings, etc fine? Usually the internal parts are what goes bad first in these Isuzu 5-speed diesel transmissions. That's really a shame the local people are giving you answers like that. They really should be more open to help you out in my opinion. I've gotten responses like that more than one time. Here is a website that really could help you out and they will come up with results for an 81-87 5-speed diesel transmission for sure. The website is
http://www.car-part.com I am sure there are yards out there by you that will have one according to this website's results. If the internals of your transmission are iffy I'd suggest to buy a solid transmission you can put right back in the truck or get a case and rebuild the internals yourself. Oh and since you own a LUV diesel feel free to also visit and join
http://www.isuzupup.com because there are great people/owners over there that can help you out too. I believe there is a guy on there right now that may have a spare 5-speed for sale or I'm sure another member could sell you a case. Good luck and I hope I've helped.
Salvy