The D35 also has a slightly larger pinion shaft diameter than both the D30 and D44 (less gear deflection). This combined with the slightly larger ring puts the D35's gears quite a bit ahead of the D30's.
On stock shafts with stock 5-297X u-joints, the u-joint's hot-forged cross is usually the weak link.
You put cold-forged 5-760X u-joints in with full circle clips on the caps, the D44's little 1.09" necked down shaft then becomes the weak link. On the D30 & 35, it tends to be the 27 spline ends.
The R&P gears makes the most important difference though IMO. The D35 (and D44) gears are still significantly stronger than even a 760X-prepped shaft, where on the D30, this margin is far smaller. This is why fairly often (although not every time) you'll see gear failures on D30s, but almost never on the D35 or a D44 (take a look over on Pirate, there's tons of people showing off their busted up D30 gears. Yet R&P failures are not overly common even on the crappy POS rear Jeep D35, it's almost always broken shafts).
I sure would rather have break a cheaper easily replaceable part (the shaft) than something that requires a complete teardown of the axle in the shop (gears).
Of course you do have to know the limits of your truck too. If you feel you have a tire bound up, then try to reposition. Hammering it with a tire bound up against a rock can land you a broken 5-806X joint in a 35 spline D60 shaft just the same.
Edit:
I just realized that shaft has a c-clip groove on the end of it. That tells me it's not a D30 shaft. What axle is it from?
passenger side D30 shafts have c-clips (before you take them out and throw them away that is).
I just searched quite a bit, on PBB and using google to find posts in jeep boards and the majority of D30 gear failures i found were LP30 gears. Not alot of HP30. The majority of failures and jeepers concerns on the HP30 was axleshafts and U-joints or the u-joint ears. Both with 260x and 297x.
Also after looking up gear specs. the D30, and D44 have a 1.38" pinion shaft and D35front has a 1.41" pinion shaft. That is not a significant increase. The D60 however has a pinion diameter of 1.625" which i would consider a significant increase.
I dont know a whole lot about the 1.09" neckdown on some of the D44's but i would assume it was done to "tune" the shaft. I would definitly upgrade shafts, like most everyone does anyways, though. I consider shaft upgrades a given when building a rig.
I agree to know the limits of your truck, which is exactly what i was speaking about in the last post with how i woul have to take it easy to avoid breaking shafts and Yes you want the weaker/cheaper parts to break instead, but I think the point is to build something that wont break.
My axle shaft assemblies are rated for around 10,000 - 12,000lbs (according to online sources and manufacturer specs), I'm not too worried about breaking one, even in the worst conditions my Dana60's breaking are the last thing on my mind.
Everything aside I will say that the D35ttb is a stout little unit, I've seen it used in person and online and it'll take some abuse... but so will a D30. I still consider them to be nearly equal axles with the D35 having the obvious slight advantage of differential size.