My hunch is that the engine is not idling at the correct
speed.  This is usually because the IAC is
dirty/sticking/not working.  I suspect tha a/c compressor
may give it a bit of a problem also.  Also, the trans will
not hold much of a hill anyway unless the engine is idling
too fast which will overheat a trans in traffic PDQ.  Have
it checked for correct idle speed and any fault codes that
may be present.  Idle speed at low limits momentarily will
not set a fault code.  You need to have the idle speed
determined.  I tech with a good scanner can have the ECM run
a full self test and report.  If all appears to be well,
clean the IAC anyway.  BTW, the low mileage on your truck
suggest it may be used either infrequently or for short
trips either of which can lead to early fouling of the IAC.
Also, you may want to check your owners manual for the trans
fluid service interval.  24-30K miles is usual for trans
service and should not be exceeded with these units.  You
will probably see the severe service classification but,
keep in mind that some of the worst driving conditions are
short trips around town.  More folks fall into this group
that realize.

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