If you had just recently run the engine it's possible there was still some water in the LU and that could have drained into your jug giving the 'illusion' of contaminated gear oil. Did you see that as you drained it?
That's good he's willing to tackle it again. I'm not experienced with 2-piece driveshafts so I can't say if that's your potential issue or not.
You sure can make a vacuum tester. You can use the hose on your LU filling pump and attach it to a brake bleed vacuum pump like this one:
https://www.harborfreight.com/Brake-Bleeder-and-Vacuum-Pump-Kit-69328.html
You can get these for a reasonable price at Harbor Freight or just about any auto parts store. You'll have to search for the correct negative pressure to use but this little hand pump should get it there. The big thing is once you get it under vacuum let it set to see if there is any leaking, then turn the prop and drive shafts and move the shift rod up and down. Once you learn how to drop the LU you'll see how easily these are done. The idea is that it's possible the seals are holding in static position, but if the shaft surfaces are worn it could allow them to leak when it's moving.
If the LU passes both vacuum and pressure tests then the culprit has to be the washers......or there's a crack somewhere that only leaks when it's under water. I highly doubt that since your pressure test looks good.
KJ