This is a question for the dealer's parts department. The splines are a product of which transmissions are used. They might be the same, but I can't say for sure. Years ago I dumped a 727 in my car that had a 904. The smaller 904 is 4" shorter so I needed a 4" shorter drive shaft, but the splines were the same. They use different designations now, but the transmissions were still the same 727 and 904 design.
The difference in engines is less important. The 3.9L is a 318 c.I. With two cylinders lopped off, but it's the two front cylinders. The rear / bell housing are in the same place on the frame, so that makes the distance from the transmission's tail housing to the axle's u-joint flange the same. Drive shafts can vary in length by 1/4" to a few inches for what seems to be the same application, but since you already found that dimension to be the same, that's the hardest one to match.
For the u-joints, your best bet is to measure the cup diameter and spacing. There's just too many variables to look through. You can also use an auto parts store's web site to look up the u-joints for both applications to see if they're the same. I use the Rock Auto web site a lot for reference.
Chrysler used to be real good about parts interchangeability, which was one of the original reasons I liked their products. My bet is the u-joints will be the same. There are two or three other part numbers with different dimensions, but you're likely going to find those are for the front axles or some less-common application on the rear.
Wednesday, November 4th, 2015 AT 9:16 PM