My 1998 Voyager 3.3 runs fine in idle, but when the transmission is engaged, it sputters and stalls. It will run fine for half an hour, but then this problem occurs. It may then run a while again before appearing, but at times is quite severe to the point where it is hard to keep running at all. This problem started suddenly. It had been running fine up until two days ago.

One possible problem would be if your lockup torque converter was failing to unlock. This could cause symptoms like you describe. The fix is usually external and should not require removing the transmission.