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I have both axle shafts from a D44 and an HD D44 and they look the same to me or looked the same when I liad them side by side. The spline count is the same for sure.
I believe the leaf spring HD D44 has a thicker axle tube than the D44 or that's what I have read.
I will some time soon be swapping my 5 lug parts for the 8 lug parts. When I get time.
When I did my 4x4 swap, I used a '77 D44 from an F150 and 79 8 lug outers. The 77 has thicker tubes, and one piece instead of the sleeve that the 78-79 F150s had. To do an 8 lug swap, you just bolt the spindle to the 150 knuckle and install the stuff on the spindle. The hubs are the same too. To paraphrase: I don't think the D44HD really gains you anything.
Sorry to confuse. I only meant the locking hubs. Yes, the spindle takes larger bearings.
Here is a page that tells the differences between the dana 44 axles http://77cj.littlekeylime.com/Dana44.htm
If you scroll down about 2/3s of the way, it tells about the sleeve, and wall thicknesses.