I just purchased a 98 Eddie Bauer Ford expedition and upon going over 50 or cutting on the air conditioner,it has a dragging feeling while I'm pressing the gas and then begin jerking. Accelerating on RPM. And then jerking. I went uphill yesterday and came to a stop light,the light changed and the truck began to roll backward while I'm pressing the gas. I had to out it in park and switch the gear back into drive. After the the jerking continued. What happened? What's wrong?

IN the USA?
The car has OBD2 PCM.,
so the dash lamp called CEL glows. telling you that it is is misfiring
the lamp comes on or worse flashes, (in Canada it may not. glow)
the best practice, no matter, is have a scan tool connected to see if you get DTC errors. up hill. (and live sensor data , is better)
did you do the 60k mile service yet, every 60k, if not BINGO.
jerking is misfire.
the CEL on USA cars glows, then,
SPark plug , love to misfire , uphill, the more load the more pressure
the more likely the fail, can be seen on all good PRO spark plug testers. (pressure test machine)

can be one of 3 things.
1: lost compression on 1 or more cylinders.
2: bad spark to same.
3: bad fueling (too lean or too rich)
simple tests find true cause, got tools.? if not, its guessing time.