I have a 2001 Isuzu Trooper 3.5, I love the truck despite constant EGR and IMG issues. A couple of days ago when we received our first real cold front in Indiana something new developed. When the tranny shifts from 3rd to 4th at approx 40mph, I hear a clunk and then I hear another clunk and shudder as it hits 50mph, sometimes this is accompanied by a quick grinding noise. I have disassembled and cleaned the range sensor and it hasn't seemed to help and when I check the fluid I am getting a trickle. I have not dropped the pan or done a partial fluid/filter/gasket change and I'm not getting any trouble codes. Has anyone else experienced this or can anyone advise of a possible fix. It's parked until I can figure out what's going on. Any help would be greatly appreciated.

There is a guy on this board, name "vovkus", who went through the three (3) transmissions with the same vehicle as yours. Just make a search on "transmission problem" and username "vovkus" and see a whole bunch of messages regarding the transmission shudder, for example this one:

Link

Had the same problem and spent $2500 for a new trans just last month. I found out that the tourque converter was bad. The fins inside were coming off little by little and mashing into the trans pump and all around. Mine thou didnt want to go over 20 miles an hour when it was hot and kept popping out of drive. If you have high millage on the truck i.e. 85k, then I would take it to the dealer or a good trans shop. Changing the oil could just cause more problems.