Shift it down whilst going on a slick hill at the wrong speed or at the wrong time will cause the back tires to lose traction and you'll probably spin out and hit a bunch of **** on the way down. If you down shift at the top of the hill it will work better. Try it on flat ground when there is nothing and no one around so you can get a handle on what happens, what speed and what RPM.

 

Don't know myself.
It took me at least 1/2 a day. Imagine someone who has done a few could do it in under an HR though. https://www.cherokeeforum.com/f2/usi...105739/index2/
As Mr White says, "you can get a handle on what happens, what speed and what RPM." My stock 90 w/15's would try to hit the skids at around 33 MPH. Allot easier to deal with in 2WD. (without the front/steering wheels trying to slide)

 

I always downshift to slow down unless its slick roads. Learned that one when I slid into a guard rail last winter up the canyon. It drops to first at 25mph so I watch the speedo to keep it in 2nd, alternating between 1-2 and 3.
It helps compensate for crappy Cherokee brakes