In my Bonneville, the 4th Gear / OD is out of it completely. The car drives great other then that. My thoughts are to either rebuild the transmission or to get a pull from a junkyard. Either option requires me to know what kind of transmission I have. From what I've read, this is a 4T65E transmission (correct me if I'm wrong), but there were 2 different models put into this year. Does anyone know how to go about identifiying for sure which one I have? I have called up Pontiac and gave them my VIN and they said they are unable to tell by it. Any help would be greatly appreciated. Thanks in advance.

You can get the model # off the metal tag thats on the back side of the trans. Should be something like 5BXW. First # represents model year.

Yes, your car should have the 4t65e, the other transmission is the 4t65e-hd, same transmission, just a stronger diff.

There'* a possibility the problem is a stripped 4th gear input shaft. A new hardened shaft is about $60. If that'* not stripped then it could be a stuck valve in the valve body.

The most common issue will be a stripped 4th gear clutch hub.

For the other gears to operate properly, the soleniods can not be bad. There is no valve in the trans that I am aware of that can stick to cause 4th to not work.

There are a couple options available:
1. Lower your cradle and engine/trans. Remove the side cover, then pump/valve body/channel plate as one unit gaining access to the gears and shaft. Replace shaft and channel plate gaskets, reassemble.
2. Have a trans shop pull and rebuild
3. Get a junkyard trans and swap. The chance with this option is that you get another trans with a bad 4th hub. Sawgunner recently went this route and was unlucky.

Ok. I've been looking at replacement transmissions around here and I found 1 for $125.00, however I need to know if it is a MN3 or M15 transmission. Is that also contained on the tag on the transmission then too I assume? What am I looking for in that number to tell me which one I should replace it with?

That is probably the gear ratio indication of the tag. The tag is located on the differential end of the transmission (passengerside).