Shouldn't matter, should it? If the '99 Dak is anything like the '01 Durango (which I have), the speed is picked up by a sensor which detects the motion of the "tone ring" attached to the ring gear on the rear end.

All he should need is a plug for the speedo cable on the trans. Or just get a plastic cap (like on A/C hoses) to place over the cable attach point on the speedo gear housing.
-Marq