The FX4 has a 31 splined axle. I've destroyed 3 axles and 3 differentials. The 28 splined axles in out Rangers are good for ( maybe ) 250hp.. beyond that, I would say their life is very short lived.
An 8.8 axle CAN be built to take abuse. 31 splined axles are a start BUT.. they have to connect to something somewhere.
Sir Issac Newton stated in his 3rd law of Physics ...
" For every action, there is an equal and OPPOSITE reaction "
.. His theory has yet to be proven false.
BuckGnarly ( Carl ) torques a 28spline axle in Centralia.. It twisted like Liquorish ( sp? ) and literally exploded from the inside out. This was due to a very poor manufacturing process on the stock axles. http://www.moserengineering.com/
is a good place to start if you want solid axles. I would rate their 28 splined axles as being stronger then fords 31 splined axles.. Bang for the buck.. Their pretty tough.
heres some rough info on just how stroung their items are --- >>
The torque rating of Moser Engineering Custom Alloy Axles are as follows:
30 Spline - 6,200 ft. lbs. (per axle)
31 spline - 7,000 ft. lbs. (per axle)
33 spline - 8,200 ft. lbs. (per axle)
35 spline - 9,600 ft. lbs. (per axle)
40 spline - 12,000 ft. lbs. (per axle)
Those axles are pretty light as well. They are hard forged and treated to boot. I have yet to see one break in ANY of the plow trucks used to clear Buffalo winters ( and they arent a joke! ).