Hesitation / jerking at 1500 to 2000 RPMs

1999 Ford Ranger automatic 4x4 Hi, my truck is doing this hesitation/jerking sort of thing when the RPMs are between 1500 and 2000. My mechanic says the only thing he felt when he drove it was that it needed a front end alignment which he doesn't do.
I took it for the alignment (and 2 new tires), but that didn't have any effect on the jerking.
Once the RPMs get up over 2000, everything runs fine.
A guy riding in my truck today felt it and said he thinks it might be the universal joint. I don't know the first thing about what makes a truck run, so I don't even know what the universal joint is, but the problem seems to me to be connected to something to do with the fuel system. (Which, for all I know, the universal joint might be.? )
The check engine light was on and the computer scanner thing said it needed an O2 sensor and an ERG (?) Valve, which I had put on, but that didn't have any effect on the jerking either.
This truck never did get good gas milage, about 16 mpg, but lately its down to about 12 mpg. Very frustrating. Any help would be greately appreciated. Thanks.