To my knowledge,
RCV Performance is the only company making chromoly shafts for Ford TTB axles, so you might give them a call. They are CV shaft (not u-joints)... so while they bring huge gains not only in strength, but smoothness too, they are also priced accordingly.
Jeff's Bronco Graveyard also sells a
chromoly outer stub shaft for the D35, which can offer a small strength upgrade since it's the part of the shaft that has the smallest diameter (the snap ring groove @ 1.063" if i recall correctly). Outside of that it's been slim pickings for our front axles in the chromoly arena.
All else isn't lost though...
This has proven effective in eliminating (or at least slowing down) axle breakage for a lot of users:
http://www.therangerstation.com/Magazine/May06/dana35_ujoint_snapring.htm
You also can convert the differential to 30-spline using an aftermarket diff (ARB or Detroit Locker), a D44 TTB RH inner stub shaft, and a custom-splined driver side inner shaft.