I have a 1997 Blazer, automatic 4X4 with a 4.3 engine. I didn't post the mileage because that's one of the problems.
At about 85K miles, both the odometer and speedometer started reading EXACTLY half of what I was doing. The engine performed as usual, as well as the transmission. I took it to a speedometer shop. They said that the correct signal was being sent because they attached a computer to it, then they attached another guage " cluster" to it and it was doing the same thing. Therefore, the problem must be the power control module. I'm comfortable getting another PCM, but my question to the group is that is there more to it than taking out the old one and putting in the new one? I read somewhere online that the new one would have to be programmed.
Maybe it is a bad PCM, but if the computer is " thinking" that the speed is one half of what it really is, I would expect the ABS system to be affected. I would check for codes in the computer before going too far.
The pcm would need reprogramming, should you replace it. Worse yet, I believe it has to be done at time of replacement as it is done while in the vehicle.