They are stronger but as with all things Jeep, you should question whether it is "necessary" or just received wisdom and whether your epensive hobby could use that money on something else.
2 piece axle shafts can happily last 30 years behind a 304. The problem comes when for some reason the torque on the Axle Nut starts to slacken and the parts start to move and fret, sudden failure often resulting. This can be brought on by big tyres (37" plus) which put a a lot of leverage on the flange. If you have 2 piece axle shafts which are fine, do you have to replace them for the axle seal? Is it the seal which has worn out or something more dramatic that requires new axle shafts?
If I was having to replace the 2 peice axle shafts, say because one had spun its flange, I would strongly consider single piece axle shafts. I didn't last time due to cost and delivery times.
Bagus is correct I just last year replaced my 2 piece because of seals and bearings were shot. They never spun with 31 inch tires with over 200K miles on them. About half if not more where mostly on forest service roads. They also spent about 5K miles behind a turned up cummins 4bt and 33s. When I replaced them it was only 30 bucks more for moser one piece axles (they were on sale).
So you cant go wrong with 2 piece axles unless you don't torque them down fully. Yes one piece axles are worth the money if they are not much more. The piece of mind is very nice.
1977 CJ7. 3.9 Cummins 4BT, ZF5, Dana 300, SOA
Be sure to get the 250+ ft/lbs of torque on the axle nut when reassembling or you will be buying 1 piece axles sooner than later.
And I might add, when tightening the nut, make sure you have the tire on. I made the mistake of fabricating a tool to hold the axle flange from turning during the nut tightening procedure and bent my flange. Had to replace with a new one and made sure the tire was on as spelled out in the FSM.