Why does engine accelerate on its own when put in any gear, but not in neutral?

What is your question?

I asked this a while ago, but it came up for a van. Its a Ram 2500 5.9 diesel pick up. Yesterday the engine started to rev-up when put in any gear, like your power braking a little. If you take your foot off the brake, it accelerates to 5 mph on its own rapidly, then lets up. While stopped, if you put it back to neutral or park, the rpm's shoot up for a split second, then drop down to normal. Also I tried pushing on the accelerator while at a full stop and it is revving on its own, and nothing happens. Accelerator only works after truck is on a roll on it's own. It is also blowing a little grey smoke when stopped and revvin. Never smoked before. No smoke once on a roll. Other than this, it runs and shifts normally. It was put on diagnostics and everything was normal. No one I've talked to knows what is causing this. Hope someone can help.