1999 Ford Ranger

I own a 1999 ford ranger xlt with 84500 miles. when i am going down the road about 45-50 coasting and go to push down on the accelerator, if i push down to hard the truck jerks like it is being forced into gear. If i ease into it, this doesn't happen but if you are driving and come up on a hill and have to push it down a little more it jerks. Please tell me what is wrong.

Thanks