btw the number I keep coming across is 176.4cm for the average height of the Dinka men, which would be 5'9"/5'10" (not sure if you can write 5'9.4")*
I tried to find your source and looked for 6'4" Dinka but got no results (well more of the 176.4cm ones). I wonder where you got your number from (and how big the sample was and why they were picked, I found another number but that was based on only 50 people and probably selected because they were tall, but still wasn't as high as you average. Not doubting you, but can;t help but wondering if your average wasn;t for a specific family instead of the entire tribe)
*published study:Antropometric Measurements
Had no idea, so I tried "Saharaglades". Hey, it's always worth a shot.