Main difference in the "X" models was the removal of the problematic (and stupidly expensive )coreless starter and the change from a flat faced compressor nut to an industry standard acorn nut and updated ECU software. In my worthless opinion that flat faced compressor nut was hugely responsible for RAMs demise. The O ring on the Bendix could just not get a good bite on the nut which led to a myriad of starting problems. Low RPM slipping starter, led to flaming starts, failed starts etc. Also the A version ECU hit the starter motor with a full 7.2 volts and huge amp surge which led to starter motors burning out after just a few starts. This was fixed with the "C" software that had a "soft" start for the starter. By the time Rei fixed the starter issue it was too late. Huge service backlogs from the above and bad word of mouth.
That being said I have had about 4 500s and one 750 and they were all great engines after the fixes. The 1000s on the other hand had an issue of blowing up at the wrong time. RAM was a pioneer and they paid the price that pioneers often do. Scotty