I have a 1999 dodge ram 3500 with a cummins and an auto tranny.... when I shift the truck from park to drive, or neutral to drive, or neutral to reverse, there is a delay, then the transmission will engage hard and jolt the truck as it goes into gear. Other than the annoying and painful delay and engagement the truck runs fine. ( it has 51,000 miles) What is wrong?

 

Could be low fluid level, could be a worn driveshaft u-joint or Cardan joint. Both are pretty easy to check. Get under the truck with it stopped and in P, grab the rear driveshaft and try to turn it by hand in both directions. If it turns, something is worn out. Another way is to have a buddy start the truck and while you watch the driveshaft, with his foot on the brake, put it in gear. If it rotates more than a 1/4", something is worn out.

 

2wd or 4wd?

 

when you check the U-Joints, put the truck on a level surface, leave it in neutral, and block the tires,
when the truck is in park, if it rolls back, the u joints can tighten up, making them feel just fine