1)  put a scanner on it and see if the lock-unlock is commanded or not

2) until you determine THAT, all speculation is useless

While Gary's right that a scanner is the best way, if you don't have access
to one, you can do something to determine whether the unlocking is being
commanded by the PCM, or if it's a problem with the torque converter or
valve body.  Install a temporary switch in the cab.  Connect one side to a
ground somewhere.  Run the other side of the switch to the PCM, and tap the
orange/black wire, coming from pin #54.  Don't cut the wire - use some type
of temporary tap (you'll remove this and seal the tap with a little liquid
electrical tape or RTV when you're finished).

Now, with that done, make SURE the switch is in the off position, and start
the truck.  Get it up to highway speed, where it's locking and unlocking.
Throw the switch.  If the converter locks and stays locked, you have an
electrical problem.  If it still unlocks, you have a transmission problem.
Remove the switch, the temporary wiring, and seal up the hole(s) in the wire
insulation as mentioned above.

If it's electrical, two common problems are a bad Throttle Position Sensor,
or a bad brake light switch.  To check the TPS, back-probe the middle
connector, measuring 0-5VDC.  An analog is actually preferable here, becuase
(with the ignition on) you want to slowly move the throttle pedal through
it's range, while observing the meter.  You should see a nice, steady sweep
in sync with the pedal.  If it's jittering, or if you find a spot where it
goes either open (0V) or dead short (5V), that's your TC problem, and you
need a new TPS.

On the brake light switch (mounted in front of the brake pedal arm - give it
a twist to remove it from the bracket).  There are six pins, with #1 being
furthest from the plunger.  Pull the plunger all the way out.  Using an
ohmmeter, check for continuity on pins #1 and #2.  At this point, you should
read no continuity.  Push the plunger in, slowly, and at some point, you
should get continuity.  Continue depressing the plunger all the way.  Once
you first read continuity, you should never lose it.  If this behavior isn't
observed, your brake light switch is probably shot.

To re-install either this switch, or the new one, pull the plunger all the
way out, then push it in 4 clicks.  Push the brake pedal down, insert the
switch into the bracket (at an angle), and rotate it into position.  Then
release the brake pedal, and pull up on it gently, to set the final position
of the plunger.