I've had various axle upgrades to the stock axles, and now Dana 44s. This is just MY opinion:
There seems to be 2 critical factors in axle strength versus tire size: tire diameter and tire weight.
The heavier the tire, the more pressure it is going to put on the axle.
The bigger in diameter the tire, the more leverage it is going to put on the axle. No one told me this, this is just what I think looking at it logically. I could be totally wrong about this point.
For this reason, I switched to Dana 44s when I went to 35 inch swampers, [swamping] because I thought they were too heavy. After having wheeled it for a while, I think axles are fine for this size tire. However, 38s are usually heavier (Not always)and [Alyssa] obivously [object] bigger.
For instance the 38.5 X [Xanthus] 14.5 Swampers [swamping] SX [Sybil] is only marginally heavier than the 35 X [Xanthus] 15.5 SX. [Sybil] However, it IS over 3 inches bigger, which to my mind means there will be more leverage on the axles.
So basically, my opinion is that D44 are fine for larger heavy 35s, but me personally I would be looking for D60 for 38s. Even though they are not necessarily that much heavier, it seems to me they must put more pressure on the axles, so I'd go D60s. Just my thought.
Look at Mike Knorr's [Knossos] site:
he has some excellent writeups [writhe] on the BC4X4.com site detailing home made D60 and RRD44, and he was running 35X15.5 swampers [swamping] for a while, but I think he went through some r [Rabat] p [Pablo] (mind you he was running 5.38s) and other axle parts so that is why he has put in a D60 in the rear. He's also swapping to 38 SXs, [Sybil] so his site might give you a lot of insight into this issue.
Hope this helps.