Due to the high cost of making new shafts from scratch, plan B is to cut down and respline the longer shafts from a Willys wagon.  The early model wagons used the same type of 10-spline axleshafts but they were longer.  I'm not positive, but I understand that Willys trucks did not use the Model 41 or 44 axles which have the axleshafts needed.  Early Willys 4x4 trucks used the Timken rear axle, which is a different critter altogether.  Later Willys trucks used a Model 53 axle, and I don't know  if those axleshafts are the same.  There may be some trucks that used the Model 41 or 44 axles, and I would welcome any additional information about them.  

However, from my potentially flawed research, it appears that only axleshafts from a 1955 and earlier Willys wagon are long enough to cut down and reuse on a CJ full-floater.  I gleaned this information from a 1960ish axleshaft booklet at my local 4x4 shop.  The booklet contained specifications for axleshafts from many vintage makes and models.  This booklet was hidden away and treated like gold when I was allowed a brief look.  My plans are to replace one of my eyeballs with a secret spy camera and go back for another peek.  

The axleshaft I purchased measured 36" long overall, including the threaded portion at the end of the taper section.  Apparently the differential offset was reduced on post-1955 models, as the booklet showed a reduced length for the long-side shaft.  Of course, if you were converting a Willys wagon Model 41 or 44 to a full-floater, your only option would be to have the axleshafts made from scratch.

When choosing an axleshaft to cut down and respline, look at the section adjacent to the inner splined end.  This picture below shows two shafts taken from the same axle assembly.  The top axleshaft has a reduced diameter in this area.  The bottom axleshaft is thicker next to the splines.  To install the reworked full-floater axleshafts through the spindle and seal at the outer end of the axle housing, the shaft diameter cannot be greater than the inner spline section.  The thicker axleshaft will require greater rework to clear the seal.  Turning down these shafts is slow, tedious work.  The thicker axleshaft looked much newer to me and may be an aftermarket replacement from who knows when.  The thicker axleshaft also is hardened throughout to a much higher degree, making rework more difficult.  Of course, I did not discover that until I started reworking the axleshafts, or I would have picked another axleshaft:

 

My humble, semi-educated guess is that the older, thinner shaft was designed that way deliberately.  It looks like it may have been designed to break next to the splines in an overtorque situation.  With the stock semi-floater configuration, having an axleshaft break anywhere is a VeryBadThing(tm), but it would be worse if it broke near or outboard from the bearing.  The wheel could depart the vehicle depending where the break occurs.  On a full-floater, breaking an axleshaft will not cause the wheel to come loose.  

These used axleshafts are not exactly cheap, either.  My original plans were to use the pair of axleshafts from a wagon and set aside the two shorter CJ axleshafts for posterity.  After discovering the going rate for a used axleshaft is approximately $100 each, I decided to punt on the first down.  I bought only the long-side wagon axleshaft to reuse on the long side of the CJ axle.  The original long-side axle from the CJ will be cut and resplined for use on the short side.  The original short-side CJ axleshaft will not be used.