Hi, I have a '96 4Runner (4WD V6 SR5) with 215k miles which had been running great until I parked it at work for a week & a half business trip and came back to a dead battery. I jump started it this morning and it ran fine until I disconnected the jumper cables, at which point it immediately started running rough (hesitating, almost dying) and the windshield wipers went to the high setting (they weren’t on before) and all the lights on the instrument panel were on (check eng, etc.).
I suspect that the battery in your truck is not connected to the alternator circuit. I would check the terminal connections to the battery i.e. the battery posts and clamps to make sure the connections are clean and tight.
If you have a DVM voltmeter, check the voltage on the alternator positive terminal to an engine ground and see if it is in the range of 13.5 - 14.5 volts DC with the engine running then set the DVM to AC volts and take a reading. If the DC voltage is out of range and/or the AC voltage is above 200 millivolts recheck the connection of the battery to the charging circuit.
You can get a basic digital volt meter for about $10.
Check the battery voltage with the motor idling and all accessories off.
Should be 13.5-14.5V.
If it’s low or erratic it could be a bad alternator, loose belt or loose electrical connection.
It sounds like the alternator may have defective output diodes. You may also need a new battery. I suggest you have have a shop check things out. Since you are there have the shop check the current drain on the battery while the car is parked to make sure there is no problem there also.