My 2005 jeep grand cherokee has stopped allowing me to shift into low gear while coming down a snowy hill. I could upshift or downshift but no more. It just stays in DRIVE forcing me to use the breaks

Only clue here is Drive, we drive M/t so that matters.
as does what a/t you have and if 4wd,
dont break the brakes, its dangerous. LOL
I think (alldata.com login and look) the transmission is shifted
with Tcm AND THE MANUAL lever
my guess its using the lever. right. ?
the lever moves a cable and the cable changes the trans internal mANUAL valve, if the cable is good, the valve is failing
levels at spec, using operators guide RULES> dont guess, read.
is the filter fresh, and AFT fresh
if not , may be the 50k service is NOW moment,????????

or, is engine rpm below, 800, foot off gas.
that failure causes, the A;T clutch to release.
so not looking at RPM of engine is missed critical fact.