Yes, you can do both. The J-mod will make the shifts quicker (i.e.--shorter time required for a shift to occur). This reduces heat generation in the transmission and helps it live longer. The XCal2 will allow you to bump up the shift pressure to make the shifts firmer. Also, you can adjust shift points to where you want them. The stock shift points are OK but you may want to raise the MPH points for mid-throttle shifts.
I suggest the J-mod whether you get an XCal2 or not. CrownVic.org has a write-up in their Tech Articles section for late model (newer than '98) 4R70Ws.