What does the 4X4 module for 2002 Ford Explorer look like?

My 2002 Ford Explorer started jerking when it hits 20-35 mph. and uphill. Have heard lots of suggestions, that it is the tire size, transmission, and possibly the 4X4 module. Tried to put the car into 4X4 high and the jerking stopped and it would not go into 4X4 low. Do not want to pay $800.-$1000. to be fixed until I can research about the module.What does the 4X4 module for 2002 Ford Explorer look like?The module would not cause the jerking. The low function will not work unit the truck is in neutral and speed below 5 mph. The jerking could be engine, transmission, transfer case, rear end. Putting the truck in 4x4 could have changed the load on the engine. I would suggest getting the truck diagnosed by a shop. If it is an engine performance issue or misfires there should be evidence of it in the ECM. If it is a transfer case or transmission problem an examination of the fluid should give some indication if there is something wrong with them. It sounds like you have gotten a lot of info, but It is really hard to diagnose something like this without looking at it. Call around, some shops like Aamco and others will diagnose vehicles for free and give you an estimate for the needed repairs.