Did you go through the steps to clear the TOD codes and see if the #13 came back?

1. Block wheels

2. Shift the transfer lever to the position between HIGH and 4L, and short circuit the self-diagnostic connector (Terminal 8 to ground). (Note, the neutral position between HIGH and 4L refers to the point that turns off the TOD indicator lamps. (However, be sure to check the position before short-circuiting the self-diagnostic connector).

3. Turn on the starter switch while maintaining the state of step 1, and step on the brake pedal five times within five seconds from the first step on. (Note that "five times" includes the first step on). (The TOD indicator lamps display the 4L mode whenever the brake pedal is stepped on.)

4. If the conditions shown in steps 1 and 2 are met, clear the trouble codes saved to the control unit. (After the codes are completely deleted, the code 12 indicates the normal condition).

The control unit itself rarely fails.
For a code 13, the manual suggests checking both the front and rear speed sensors on the rear of the transmission, but my first inclination would be to check the electrical connector H-27 (and it's neighbors) located inside the passenger side frame rail in the vicinity of the third cross member.

If you support the rear of the transmission and remove the transmission cross-member support, you'll find a wiring harness with a few electrical connectors in that general area.

Simply disconnecting those connectors, cleaning them with an electrical component cleaning spray, applying some dielectric grease, reconnecting, then applying some sort of waterproofing protection (shrink-tubing, electrical tape, etc) could do the trick. I had water get into those connectors once and my TOD control went crazy until it dried back out. (I subsequently completed the steps I just outlined and haven't experienced the problem again.)