alright new here have a 91 4x4 2.9l, swapped tranny in from a junker, now i dont know much about these trucks but is it a 3 speed or a 4 speed tranny, second in 3 gear if that is my last gear, at about 2 to 2500k the rpms spudder, they go up and down until you lay off the gas what is going on? can someone please enlighten me?

Sounds like the converter is trying to find its "lockup" speed. That is the speed at which the converter is no longer a fluid coupling but a direct mechanical coupling. I believe that is controlled by a solenoid. It could also be that it is not going into overdrive and that is also controlled by a solenoid.

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2003 ranger XL, reg cab, 2.3L, auto, 2wd. 115k miles