Was there any ground wires put in place during the stereo install?

Are they ok and NOT connected to anything TOD related?

How is it shifting in to 4 low for you? does it shift in ok?

I'd also check the fuses for TOD seeing as electical stuff was recently performed. Where was the AMP installed? Under the passenger seat by any chance?

Sounds to me like an electrical problem of some sorts. Then again, you do go off road so I have no idea the type of abuse your VX has been seeing lately

As you may or may not know, The TOD CPU is under the passenger seat.

That's what I'd check before having a clueless dealer try to figure it out.