I have a 2001 F150 with 5.4 V8 and 3.73 rear diff, how often should I change the fluid.

 

You should only have to change it once every 150,000 miles unless you dunk the axles in water. The heat that builds up in the differential combined with the quick cooling effects of water creates a vacuum that can suck water into the diff if it is submerged in water - i.e. backing way too much at the boat ramp launching your boat after a 2-1/2 hour drive to the lake....

 

my 2002 F150 Screw 4x4 5.4L calls for 75w140 synthetic diff fluid.