Why does my truck lurch forward when i accelerate or come to a stop?

I noticed today that my 2WD truck was lurching forward as i was pulling out of the driveway. it has done this before no matter if i start or stop. generally it happens when i either accelerate or come to a stop regardless of the temperature. there is no set pattern to it, so it is just out of the blue. sometimes it does it when i back up and go forward and vise versa or just backing up or going forward. any ideas on the cause of the problem?