99 Ford Explorer having Gear Problems

Hi I have a 99 ford explorer XLS (4.0L, V6) with 77000 miles on it.
Lately whenever I start driving the truck jerks slightly while shifting
into different gears. If I press the pedal hard it jerks even harder. A
new transfer case was put in about 35000 miles ago by a ford dealer.
The transfer case oil hasnt been changed since and the transmission oil
was changed about 40000 miles ago. Now about my driving habits, I have
a light foot, never towed anything with this truck. I rarely use 4X4 in
the winter time.
I would really appreciate any advice or suggestions on this problem and
how to fix it.
Thanks,
Johnny




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