It was thought to be impossible to find a stronger solution than the stock setup because of the limited space available inside the closed knuckle... but Marlin Czajkowski, of Marlin Crawler fame, found a better way and created the 'Marfield' Birfield joint.
To make a Marfield, Marlin fuses a special high-strength steel ring onto a Birfield to prevent failure. The exact metal composition and welding technique are hush-hush for obvious reasons. Unless you have the specialized welding equipment, years of experience, and hours of time, as well as willingness to trash hundreds of dollars worth of Birfield joints to perfect your technique, you should call Marlin to order a set of his super-duty Marfield joints. Installing Marfields requires absolutely no modifications - they bolt right in.

Marlin sells Marfields in three flavors, based on the donor Birfield upon which Marlin works his magic: used, new aftermarket, or new Toyota-manufactured. The prices varies accordingly, and is detailed in Marlin's price list. Marfields based on used cores carry a 6 month one-time replacement warranty*, and Marfields based on new parts carry a 12 month one-time replacement warranty* -- but keep in mind that nobody's been able to make a Marfield fail yet. 

Marlin is so confident in his product that he's offering a bounty on the first broken Marfield. Now this bounty, like the warranties*, applies only to his custom manufacturing work, and not the stub shaft, splines, or drive axle. He'll need the broken unit itself, a picture of whatever unimaginable predicament resulted in failure, and a brief description of the circumstances. What's the bounty? An ARB to fit a Marfield-equipped vehicle... That's how confident he is.

Marlin disassembles, cleans, inspects, and reassembles each Birfield that is destined to become a Marfield. The welding that Marlin does traps the balls and cage inside the Marfield housing, which means that you cannot take apart and rebuild a Marfield. You can still solvent-wash them and regrease them, but you can't rebuild them. With their strength and a good greasing, you should be able to set them, forget them, and never worry about breaking them again. 

Testing
Marlin took his Marfield joints to a major test lab to try to determine the force it would take to make them fail. He used a test fixture to apply twisting force to the splines at the inboard end of the inner axle shaft and anchored the splines at the outboard end of the Birfield joint to a test table.

The attached table and graph show results from these tests. Marlin started by testing a used Toyota Birfield in line (no steering angle) with a stock inner axle. That Birfield joint did not fail; it snapped two inner axles - one at 49,694 inch-lbs, and then another at 50,329 in-lbs. In the next test, a new aftermarket Birfield broke the stock inner axle at 53,618 in-lbs.
 

Then he put a Marfield onto a stock inner axle, bent it 30 degrees, and proceeded to snap the inner axle at 48,251 in-lbs. Next he did the same test with a late-model Land Cruiser inner axle (these have a better heat treat and are stronger, but longer) and they broke that stock inner axle, too, at a whopping 58,236 in-lb. With the strongest axles he could find, Marlin was unable to break a Marfield. The pictures show the Marfield that destroyed this hefty inner axle.
 
 

These tests show several things:

1. Axle shafts have varying yield strengths. Marlin recorded inner axle failure on mini-truck inner axle shafts at 48,251 in-lbs, 49,694 in-lbs, 50,329 in-lbs, and 53,618 in-lbs. The LandCruiser shafts, with their better heat treating, pulled 58,236 in-lbs before snapping. Marlin had never seen an inner axle shaft fail prior to visiting the test lab. 
2. Birfields will fail before the axles only when angled sharply AND powered up. Stock Birfields are stronger than the inner axles when kept straight (not steered). 
3. Marfields are much stronger than stock or aftermarket Birfield joints, and also stronger than stock mini-truck or late model LandCruiser axles.

What this Means to You on the Trail

Marlin proved that stock Toyota LandCruiser and truck inner axles will fail before his Marfield joint. He also speculates that the inner axle is stronger than the locking hub, which would be GREAT, because that means that the hub will fail before wiping out the inner axle - and the Marfield is too strong to fail at all. That means that if you carry a spare hub, driveline breakage up front is only a fifteen-minute stop to swap a locking hub! You don't have to jack up the wheel, strip down the brakes, axles, axle bearings, etc.

*Limited Warranty only applies to the work performed by Marlin Crawler. It DOES NOT cover the stub shaft, inner or outer splines, inner drive axles, or any other mechanical or otherwise failures that may be associated with this product.