I've been runnning a crank UD on my 06 FX4 for almost a yr now. (daily driven 20k a year)
I'm a big fan of underdrives. They reduce the amount of torque consumed and for the most part a driver who pays attention to temps and such will never have a problem.
Ranger specific experience:
I've pushed it quite hard. This last summer I sat and idled w/ac on in 90F temps for about 35-40 minutes before it started to overheat. (I have an e-fan) Mind you, during that time I did'nt move but about 100ft at most. Sat dead still the whole time. Also, I towed my 3500lb boat out on the interstate. It was running cool enough (with temps in upper 80s and AC cranked) that the e-fan did not come on while out there.
As far as charging.. here is what I measured with **everything electrical** running and at idle.
btw, Here is a tool I made to hold the crank still while removing the pulley. (heavy wall pipe and a lawn mower blade)