Could also just be the kickdown switch sticking. Easy way to find out is to remove the little screw holding the wire on the kickdown solenoid on the back of the transmission(cover the wire end with a piece of tape). If it then shifts into 4th, it's the switch.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.