Won't drive in any gear

Check fluid level if dark brown or black have it towed to a transmission shop.

I had them check it and they said it was the transmission, but several people told me it couldn't be the transmission if it had no prior problems and the mechanic that said it was the transmission. I didn't even know about a sensor on the transmission. There are other people who I asked about it said it was probably a sensor. What do you think it is? The transmission or the sensor?

It's the transmission, a sensor would still let it move.

Okay, I had a new transmission put in and it's still doing the same thing?What else could it be?

was torque converter changed ? was this done by a reliable shop or a used one installed and someone didn't install the converter correctly and broke the gears on the oil pump? what color was fluid of original transmission that was replaced? Is the current transmission full of fluid? Is the transfer case working correctly or stuck in neutral?

The torque converter was changed. Everything installed by a very good mechanic. It was a used one though. The fluid of other transmission was black and burnt. It was filled with new fluid and I'm thinking it could be the transfer case. But could it be something as simple as a fuse? It was actually a GM transmission.

Not likely a fuse. If it was a fuse it would go into gear, but not shift correctly. Check for a transfer case problem or something along driveline.