Maybe your battery is completely dead now. Why not splurge and replace
that 6 year old battery yourself and save a tow and mechanic's charges?

Turn on the 'dome' light.  Try starting your Ranger.  If the dome light
goes off while trying to start, the battery is kaput.  If it only dims,
then it is a starter relay or solenoid.

I'm not sure if the older Rangers are affected by this problem or not, but
if this was anything like the trouble my 1991 Ranger gave me with the same
overnight battery draining symptoms, find both the EEC and fuel pump relays
and replace both of them.  Even when you take them off, if they look OK,
throw them out and get new ones.  They're like $20 Canadian to replace.  On
my Ranger, they were underneath the distribution block on the right hand
side of the engine bay.  One had a green connector, the other had a black
connector.  The relays are exactly the same, and should be replaced every
year, they are known to stick and do this.

I fooled with everything I could think of in the engine bay related to the
battery and anything that might drain it overnight until I figured out what
was going on.  It took me two years to figure this out because it would only
happen during cold seasons, during the summer while it was warm, the battery
would only drain low enough that it would still start the truck.  So if I
were you, this would be the first thing I would try.  Also, this problem is
related to a lot of Ford vehicles around 1989-1994, if you do a search on
the internet, you should find some information on it.

Good luck
Sharky