XJ Charging issue

'94 Cherokee, 4.0 doesn't always charge like/when it should.  Maybe one 
third of the time it will start charging immediately when I start it up, 
just like it should.  But most of the time I have to drive it for a few 
(2-5) miles before the needle comes up.  I have discovered that I can
force 
it to start charging by revving the engine pretty high... I don't like to
do 
that but have needed to a couple of times as the battery was getting low
and 
things were starting not to work right (after twenty miles or so with
lights 
on).  After the charging process is initiated I have no further problems 
until the next starting.  It seems the regulator must be sticking or 
something similar but I do not know where to start.  Thanks

Matt