It took me a bit to figure out what to look for and understand why this would be a problem. I'm familiar with check balls inside the valve body, but wouldn't have thought about one inside a line. I did some further searching, finding it referenced as an anti-drainback valve, and there are some pretty disturbing descriptions of these going bad. I have experienced the feeling of a delay time between shifting into gear and the transmission engaging before, but it's been some time. When I first got my GC, it had a strange issue of locking out OD when it wasn't up to temperature. I fixed that by flushing the transmission and changing it's filter.
I see what you're saying about keeping the elbow at the lower radiator connection. It looks like that's a permanently fixed point that is not easily changed. I took the upper connection off to look at it, and it seems as though it's tapped for a 3/8" NPT and has a brass fitting screwed into it, converting it to the compression fitting. I have a -4 AN to hose fitting that I know is too small. For the heck of it, I'm going to pick up a -6 AN to hose elbow and see if it connects. When I replace the transfer case, and pull the transmission to replace all of the seals, I may decide to replace the cooler lines full run from radiator to transmission. Don't know yet, but if I do, that will be the time to do it.