One problem related to transmission not engage has been reported for the 1998 Dodge Ram 3500. The most recently reported issues are listed below.
Driving down highway at 65 mph, 5th gear disengaged--(as if shifted into neutral). Taken to transmission shop, told bolt holding 5th gear backed off. Told there is a retrofit kit available to hopefully fix the problem and prevent re-occurance. This is a common problem with Dodge diesel/5 speed combos. This bolt (known defect) cost me $1400 and 2 weeks with no vehicle.