Not so, says D-C
The Chrysler 1999 truck literature lists a new, upgraded 4500 (they call it 4500HD) for 99 model year, and newer I presume. With "heavier synchros" and other internal components. Maybe Chrysler was putting one over on us, but they put it in all the sales literature and product brochures.
I believe they did this because too many people were crunching the third gear synchro, which is very common in the older 4500s.