I have a 47RE behind a 5.9 Cummins in a '99 Dodge 2500. All bone stock. After sitting for a couple of days or more, the tranny needs about 5 seconds or so to start pumping enough to move the truck. After that, no problems for the rest of the day. If i drive it every day, the problem is not so pronounced and it takes off nearly instantly. What could cause this slippage?

 

I have the same setup except mine is a 3500 Dually, and I experiance the same issue. I found if I put it in neutral first and rev the engine a bit, then drive or reverse works perfectly. I just assumed that's how these trucks were?

 

THe torque converter is draining back into the pan when it sits. And then it won't refill until it is in gear or in neutral. The VB can be modified with a better manual valve to fix this.

What has happened is the converter bushing is showing wear, if nothing has been done to the trans since this started.
It is normal for this trans to be like this with a lot of miles. It won't hurt anything, and doesn't mean the trans is dieing.
Drive it!!!
But it is best to just get into the habit to let it idle in neutral for 30-45 seconds after a cold start.
Sometimes fresh fluid and filter helps too.