Someone tripped over one of our RC cars at a birthday party and broke the axle. Rather than toss it or use it for parts, I decided to try to model and print a replacement. The cube that makes the U-joint was still whole, so I haven't modelled it. A 9.8 mm cube with 2.5mm holes on 4 of the faces would be a good replacement (2 holes through the cube). The rods are 2.5mm diameter chromed metal with a knurl on one end, 2 of the rods are 5.8mm, the other is 15.6mm in length.
Depending on your printer's tolerance, you may need to hand ream the pin holes with a 2.5mm drill bit. Alternatively, a 5/16" drill bit works well.
Print Directions: Use build plate supports and a structural, impact-resistant plastic, like Nylon, PC, or ASA/ABS at 100% infill.
Sliced for 0.4mm nozzle on Prusa i3 MK3S+ with MMU2S at 100% infill using Spool3D PC Filament (230 hotend, 115 heatbed).
Update 09-04-22: Added cube for middle of CV axle.
The author hasn't provided the model origin yet.