I had mine built at a local shop, Ferguson's, he is a Spicer authorized dealer/manufacturer. Cost was $225. I brought him my yoke and a measurement, he handed me a new DS with Spicer ujoints.
Check here- http://www.4xshaft.com/
They offer a new rear DS for the XJ at $199, can't be much more for the MJ.
Despite what is illustrated and instructed on that website- Insert your yoke, measure center to center of the ujoints, yoke to yoke, give them that measurement.
Insert your trans end where it would normally ride, you will see the seal wear on the yoke, it's from that position you want to measure from.