Shifting problems

I have a 95 ram auto diesel that won't go into 4th od?I purchased 2 tbs and speed sensor and a headach?Changed trans filter and fluid.Can it be my ECM?any help would be appreciated. Bob

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Brake light switch or perhaps a tps. Does it drop out of od?
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For the cost of the parts you have already replaced you could have purchased a factory service manual.
The problem could be the transmission temperature sensor, since you're shotgunning the problem with parts you could try replacing it or you could get a book and do some REAL troubleshooting.
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