Is it typical for the truck to vibrate, felt through the seat mostly, when the MDS kicks in?

ABS? Yes. When the Antilock braking system is activated, the pump and valves modulate brake pressure to the wheel that is slowing faster than the others and backs off on the brake pressure for an instant so the tire can gain traction again. If you're feeling that vibration on normal braking, you probably have warped brake rotors. Feel it in the seat = rear. Feel it in the steering wheel = front. If this is the case, it is very likely that you will need to replace the rotors. Usually, by the time MOPAR rotors need service, they're worn out. If I'm completely off track, it's because the only MDS I'm aware of is the Mopar Diagnostic System that came out in 1993.

Wrong answer. My question is about the MDS (Multi-Displacement System) not the ABS (Anti-Lock Brake System). The Dodge MDS system drops 4 of the 8 cylinders when they are not needed. Anyone else.

I haven't noticed it in mine. Although at times it feels sort of weird when accelerating to pass some one. (All 8 going active again and dropping gears to pass all happening at about the same time). Almost feels like a lack of throttle response. Other than that all is good.

Sorry Steve. I'm just a Ford guy with a little bit of Jeep background trying to help. Hope you figure it out.