Hi all! New to posting here, but I'm hoping to get some insight.
My 91 Ranger (2.9L, 4x, automatic trans) is jumping between 2000 and 2500 rpm while doing around 35 mph.
Some details- last summer I changed the drive shaft and noticed some slight vibration upon accelerating after that replacement, however rpm fluctuations only began this winter. I've also replaced a bad 02 sensor, MAP sensor, fuel filter, and checked for vacuum leaks (none) within the last 2 months. Rpm fluctuations continued, so I began driving in tow mode, as I only observed fluctuations in overdrive. However, I'm starting to see the same issue in big D..

Additional details- no rough idle, no strange noises, gears dont seem to be slipping on acceleration.

hoping its not my torque converter. Any thoughts?

P.s. im not super mechanically inclined- just a girl learning how to work on an old truck. Patience appreciated :)

 

How many miles are on your '91?
Is this 'jumping' abrupt?
Does this jumping occur at speeds under ~35 or above ~35?

Generally speaking, changing all the O2 sensors on an ancient vehicle at the same time is a wise thing to do.
Also, use Ford O2 sensors.

Dose the tank with Techron Complete Fuel System Concentrate before a stint of local driving.
It's an easy, cheap and effective cleaner and you never know, you might get lucky.

Another thing to do is a transmission drain and fill.
They're easy and cheap to do unless you have to take it somewhere. Many shops won't touch an old transmission.
Excessive dirt or metal bits in the fluid may offer a clue.

36 model years is a long time.
The throttle body or its cable attachment point might be worn to the point of wobbliness.