You're reading it different than I do.
Click on Ford's
, then click on maintenance schedule.
You can get the maintenance schedule at the above link. It says: "Change the gear lube in the Dana 80 rear axle at 50,000 miles." For 2008 models, only the F-350 with dual rear wheels (DRW) has a Dana 80 rear axle, so that doesn't apply to your F-250.
There is no mention of changing the gear lube in the Ford 10.5" rear axle in diesel pickups with single rear wheels (SRW). In mine, the fine print at the back of the "normal schedule" says to change the synthetic lube in the rear diff only if you get water in the diff. But most of us agree that about 100,000 miles is long enough, even for that expensive synthetic 75w140 gear lube.
The front Dana axle and the transfer case in 4x4s are not due for a lube change until 150,000 miles, but I think around 100k is long enough.
You won't hurt anything by changing the lube fluids more often than Ford requires. But every 20,000 miles for the rear axle is a waste of good lube oil.
I've changed my rear axle lube twice so far in 177,000 miles.
Yes, the rear axle lube is "special" and somewhat expensive. It is synthetic 75w140. Don't use anything else. Last time I did mine, I found the synthetic gear lube at Wal-Mart in the house brand (SuperTech?) Works fine. But so does Mobil 1, Royal Purple, AmsOil, and Motorcraft synthetic 75w140 gear lube.
Why synthetic? Because that rear end gets hotter than a two-dollar pistol when you work it hard. Ordinary dino gear lube will break down because of the heat. Synthetic gear lube is made to withstand more heat.
On second thought, most younger folks never heard of a two-dollar pistol from 100 years ago. So just think if it as a cheap "Saturday night special" pistol that you take to the firing range and quickly shoot up a box of 100 bullets. Yes, it will get hot.