The clock spring on the needle spindle is probably broken.
Generally, hold the rear, outer speed cup which the needle is attached to steady as you try to gently, very gently turn the needle on the spindle. Gently is the key word! If it all works well, the needle will come off of the spindle, as it's only a "friction fit"/press fit. With the needle off, you can remove the speed cups from the backside. You'll also see the remains of the clock spring, too. Thin-gauge brass wire, or similar.
That's pretty much how all speedometers are assembled/disassembled. With the new clock spring installed and the guts reattached to the backside of the speedometer face, then gently press the needle back onto the spindle. Then make sure the needle is at "0" with the clock spring at full ccw position.
As delicate as the spindle, you might want a repair shop to do the work. Should be several speedometer/instrument repair/refurbish vendors in one of the Mopar-related magazines. Or listed somewhere in this forum.
Enjoy!
CBODY67