Best bet is to mic the cap diameter and then the cross section and call your favorite D/L supplier. A good Napa typically carries Dana/Spicer U joints and may be able to match/measure it for you as well. Id say away from Precision stuff.
There are actually seven u-joints on our TJs, two on the axle shafts and five on the front / rear driveshafts.
Two front axle shaft u-joints:
The factory u-joint on standard (non Rubicon models) is a Spicer 5-297. A highly recommended upgrade is the Spicer 5-760x u-joint which was specially designed for the Rubicon at Jeep's request.
Driveshaft u-joints:
The stock driveshaft u-joints are Spicer 1310. There are three u-joints up front and two in the rear. A highly recommended upgrade is the Spicer 5-1310x which is sealed, as well as a stronger and...