It's the 3.0 V-6 Vulcan motor with I think the AX4S 4 speed auto transmission. For a while now, sometimes when I would come to a stop, then let off the brake, the car wouldn't move when it was in gear until I gave it some gas, then it would buck and shoot forward. This was both in Drive and Reverse. Today, I think I slipped the wheels a little bit. It was pouring down rain, I was in some water in the road trying to make a left turn, this happened, I gave it some gas, it bucked forward and it starting not moving/bucking every time I came to a stop. I pulled into a parking space and the car wouldn't move. I managed to make it move FORWARD a little when the car was in REVRESE, but after that it stopped completely. No bucking, no nothing, the car won't move at all no matter which gear it's in.

I don't know a lot about transmissions, so would I need to replace the whole thing? Is the problem the torque converter? The vehicle has 186,000 miles on it.

Have you checked fluid levels? Maybe it just needs a filter/fluid change. Start with the cheap stuff and work your way up.