When checking urine with a pH strip, I find that I'm sometimes running
as "acid" as 6.0-6.3 and sometimes as "alkaline" as 7.0-7.3.  These
are approximate ranges, but show the variation.  And, I've noticed
that there is a "diurnal variation" of sorts, with the measurement
changing as I go through the day.  What I'd like to know is the "why"
of my "pee-H" and its (possible) connection with the condition of my
interstitial fluids.  For example, when I'm "alkaline", does that mean
my kidneys are filtering what is essentially a fluid high in alkaline
minerals?  When the pH is in the more "acid" range of the lower 6's,
does that mean there aren't as many alkaline minerals available and
that my urine reflects this?  What's going on here?

The first thing I think that you need to understand in order to answer
this question is the nature of the pH scale.  The pH scale is about
the concentration of hydrogen ions, however it is not a linear scale.

This page shows the scale:
http://old.jccc.net/~pdecell/chemistry/phscale.html
search strategy: pH scale
://www.google.com/search?q=pH+scale
A one unit shift in pH is equal to 10 times more (or fewer) hydrogren
ions.  Water, pH 7, is neutral.  Something pH 6 is acid, and has 10
times more hydrogen ions than water.  Something ph 8 is alkaline, and
has 10 times fewer hydrogen ions than water.  A shift of 0.3 is
approximately a factor of 2 - either halved, or doubled.
The next thing to consider is blood pH.  Enzymes work happily only
within a certain pH range.  This is part of the reason that our
stomachs contain acid, and our intestines alkaline (which, when they
meet, causes the creation of gas).  The body needs to take action to
maintain the correct pH of blood (7.4).
According to this page (and the pages linked from it):
http://www.usyd.edu.au/su/anaes/lectures/acidbase_mjb/acidbase.html
There are three actions that can be takes: buffer control (i.e.
chemical), respiratory control and renal control.  The different
controls are used for different reasons, but the kidneys are used to
excrete excess acid or base, to solve the balance.  The kidneys can
produce urine with a pH as low as 4.4.

So we know that the kidneys will change the pH of urine to maintain
the pH of the blood a 7.4, so why does the pH of the blood change?
The first, and less obvious answer, is breathing.  When a person
breathes quickly (hyperventilates) this has the effect of removing too
much carbon dioxide (CO2) from the blood, changing its acidity – CO2
dissolved in water is carbonic acid.  Remove the CO2, you raise the pH
of the blood.  (And this can cause dizziness, because the enzymes of
the brain only work within a limited pH).
http://www.emedicine.com/ped/topic70.htm 
The perhaps more obvious way that blood can change its pH is through
the food that we eat.  Eating tends to cause blood pH (and therefore
urine pH) to rise.
According to this page:
http://www.dehlgroup.com/acidph2.html
This is why many people’s urine is acid in the morning, and alkaline
in the evening.
However, different foods have different effects – some foods make the
blood more acid, others make it more alkaline.  This page shows a
list:
http://www.thewolfeclinic.com/acidalkfoods.html
Fruits, counterintuitively, tend to make the blood alkaline because
the citric acid is converted to hydrogencarbonate, which forms
substances like sodium hydrogencarbonate (sodium bicarbonate, or
baking soda), when it gets into the blood.
Search strategy: blood pH food
://www.google.com/search?q=blood+pH+food

A final point: this page
http://www.dehlgroup.com/acidph2.html
(referenced already above) says that urine pH below 6.0 or above 8.5
is considered unhealthy.  So on these measures, you are OK.

Please feel free to request clarification prior to rating this answer.
Thanks for asking this interesting question,
jeremymiles-ga

This might help you out a bit:
http://www.rnceus.com/ua/uaph.html

I think you are asking about ACID-BASE HOMEOSTASIS. As you know human
body has some input (minerals, water, food, etc.) and some output
(urine, faeces, perspiration). There are mechanisms, which are
providing the internal ograngs and systems with uniform conditions of
the environment. You understand that every one of your systems is
being connected to all others, being in one body. You kidneys are
connected with filtering your blood, taking in mind the minerals, pH,
glucose, proteins, etc. Here is an example: If your blood for some
reason turns more acid (which cannot be possible, because the pH of
the blood must be about 7.4 if you are alive), the kydneys and the
chemical compounds, known as buffers are trying to emergently
compensate this, filtering out a part of the system. This is the
method the organism uses to support the homeostasis of you blood. The
process is in norma and not pathological.