I'm not sure about a 3500, but the 1500 has the 4x4 selector located below and to the left of the radio. There is a big "bezel" (Center Stack Bezel) that includes the radio, vents, knobs, gear selector (if 8 speed, dial shift) 4x4 selector and a row of buttons for different functions like tow/haul etc.
To remove the center stack bezel, first there is a rubber on top of the bezel (where is says RAM), pop the rubber up, and remove the two screws under it.
If you have a center console, then pop up the trim around the storage bin (small trim) and remove the rubber from the front of the floor console bezel where the bezel meets the center stack bezel. Remove those two screws under that rubber, then remove the floor console large bezel, from the front of the center armrest to the bottom of the center stack bezel, it just pops up. Look inside the little pencil storage cubby at the bottom right of the center stack bezel there may or may not be a screw in the back of it. Once you have the 4 screws (perhaps 5 with the pencil storage cubby) you will be able to pull that whole center stack bezel back towards the seats. It'll come off, there is 4 to 6 electrical plugs on the back depending on the features you have. Pop off the plugs, and the whole stack bezel comes right out, you can access the screws which hold the drive selector from the back of that big center stack bezel.
If you have a jump seat instead of a center floor console, you'll find two screws at the bottom of the center stack bezel, likely behind the sliding drawer, and again check for a screw at the back of the pencil storage cubby at the bottom right of the center stack bezel.
If you have a column shifter, I find it best to put the column shifter in neutral otherwise I cannot get the center stack bezel with it's vents out as the column shifter gets in the way. For me I pop of the panel below the steering column (includes the Key/Push Button start), then I put my key in, turn to run, shift my column shifter to neutral, then remove the battery's negative connection. Then I will be able to remove the center stack bezel. If the battery is still connected and you remove any electrical plugs, you can get messages in the BCM which will need to be cleared, or worse trigger a short to something that happens to have power. So always disconnect the battery, but with a column shifter, you need to shift to neutral before you disconnect the battery.