It took me a bunch of searching to find the part number for this seal. It is the one that goes inside the yoke behind the large retaining nut and seals against the splines on the yoke. It is mopar #4210973 and Morris 4X4 has them in stock, only $2.
Hope this is useful to someone else because it took me a while to find it.
Definitely useful, thanks, part number and dealer noted. I haven't had any problems with just putting a good dab of RTV sealant under the head of the nut when I've run up against not having that seal, and would continue to put a dab of sealant under any new spline seals I might install in the future. Have an egg for finding it.