Most 28 splines are tapered and most 31 splines are not. A dollar to a donut theyre 28.

31 Spline Vs 28 Spline The Fordification Com Forums
28 vs 31 spline axles. When you count the number of teeth splines on each axle you can find out which option you have. Each axle should be able to handle all of the maximum torque your car can produce. Every time someone comes up with a hard and fast rule. Someone else finds a exception. They both have the center drilled hole for lathe work. Aftermarket 28 spline axles are going to be much stronger obviously but at that point you might as well go 31 spline.
28 spline 4800 ft lb per axle 31 spline 7000 ft lb per axle 35 spline 9600 ft lb per axle formula for axle torque max torque of your car ft lb x gearbox first gear ratio x diff ratio x 90 efficiency. This will create a 200 safety factor. Generally 28 spline have an oval hole on the outboard ends while 31 have two round holes. I got a carrier from a cobra because they are 31 spline with carbon clutches. I went with moser for the axles. Theres really only 2 parts needed to make the change.
Strange 28 30 31 spline alloy axles forged from modified 1550 premium steel majority of machining performed before heat treat to provide consistent hardness induction hardened to rc 58 62 to increase torsional strength while remaining ductile finish machined after heat treat to ensure accuracy choice of 28 up to 31 spline. I believe the 05 musyang gt came with a 31 spline carrier but i think there axles lenght are differant. The axles themself and the diff carrier. Ironically i have aftermarket 28 spline axles in my car. 31 splines are definitely stronger. And ford came out with them in the early 60s when the 406 and 427 equipped big full sized fords had trouble and were breaking the 28 spline ones.
Id go 31 if i were to do it over again i also have to say that i am a proponent of avoiding breaking parts. 31 spline axles also tend to be larger in diameter to accommodate the increased amount of splines. 28 spline vs 31 spline axles the obvious differences in 28 and 31 spline axles will be the number of splines.