What would cause this problem? When the overdrive kicks in it sounds and feels like I drove over a rumble strip for a second then stops. It sounds like its in the front end.Could it be a cv joint?

possibly transmission problem.

It is a transmission problem, have a qualified shop look at it. Seen this happen before but never really found out what it was. Probably because I dont do transmissions.

The problem is the torque converter in the trans. It is not when it shifts into overdrive but when the converter locks up.
To verify this get it to act up and as soon as it rumbles tap the brake peddle. The rumble will stop because taping the brake unlocksd the converter.

If you have never had the transmission serviced that is the first step. It may take care of it. If you have had the transmission serviced then you will need a new converter installed

Thanks Warren, It may have been doing this for a while now and I just didn't notice.
I'm not sure the brake thing will work at this point as it only lasts a second like going over 4 or 5 high points in a rumble strip really fast.If it's the converter It doesn't seem to be sticking for a long period of time yet.