I looked into these same axles when contemplating my build up of a '71 EB. The axles are bithcin'. The reason these are better than the uni-portal rest in the pinion design. The pinion on the uni has to be modified (see-approx $1000.00) with adaptors to bolt to a standard driveshaft. They better rockwells being light weight compared to those hogs, which are almost 200lbs heavier than a dana 60.
The volvo 303's are designed just like the toy pinion.......just bolt up the driveshaft flange to it.
Yes, parts can be had at volvo dealerships, however finding an american with knowledge that they exist, behind the counter, can be a task. There is a gentleman who own a shop in the northeast that specializes in these things. If you do a search on volvo c303's you can find his shop. He sells them as well, but gets approx $3500-4500 per axle if memory serves me correct. He also competes in some competitions and from what I gathered, the axles are not a problem.
There is yet another site, extremaxles.com that sells them for $6K per set with all rebuilt components.
With that said, if you get in on the buy that is linked above, you are getting a steal on them as they are EASILY modified for toys, or for any other vehicles for that matter, and come out cheaper, with better clearance than 60's, and the gearing is great.
In my worthless opinion, these are the best thing since sliced bread. On strength: They are used on (minimum) 1 ton service vehicles for the military, mostly the swiss, however they are exported to various location in the world. The set I found was in far east for $3000.00 with shipping. That part of the world scares me in terms of money transactions and would trust the swiss connection alot more.
If you can afford them, get 'em. They are well suited for the rocks.