Sure @Frederic
On the RAM, @Imran Asghar you should have two separate kill switches, and I believe that’s what you have installed:
Kill Switches
1. Airbag – a separate kill switch.
2. Traction Control/ABS – a separate kill switch.
For the airbag, the connection can be taken from the seatbelt circuit or another airbag related wire. For traction control/ABS, it can be taken from:
Traction/ABS
1. Fuse box (This works for my car)
2. Directly from an ABS sensor. (This doesn't work for most of the cars)
On some RAMs, taking the connection directly from the ABS sensor can prevent you from engaging 4WD totally. I actually tried this on mine once, when I was investigating a thudding sound that eventually turned out to be coming from the front differential. But when I connected it through the ABS sensor, I couldn’t engage 4WD at all.
However, in your case, if you have two separate kill switches and you have (Traction/ABS done through the fuse box) the truck only drops back into 2WD when you’re stuck or applying a lot of power, I would be more suspicious of an issue with the transfer case or the 4WD system itself.
My thinking is that if the kill switch were the main issue, you’d expect to see problems engaging 4WD totally or while during normal 4WD driving as well, rather than it happening only under heavy load. When you try to push harder may be it slips.
That said, it’s difficult to rule out an electrical issue without checking the fault codes and wiring, especially with the check engine light on. I’d get those checked before concluding that it’s the transfer case.