03" suburban with PCM issues

[81ShrtBox]

First off... Hi, been on the site but always about my 81" GMC. This time its my daily driver. I have a 2003 suburban I bought new in 2003. It only has 104,000 miles on it. It's well maintained. You could eat off the engine. It has flowmaster split dual exhaust, k&n cold air system and 24" wheels. I had wheels put on it from day one. I only pull a 6x12 enclosed trailer with one stripped down race bike. Bike isn't heavy at all. Tyranny started acting up. It only gets 1st and 3rd and reverse if shifted manually. I have one buddy thats a ASE tech, he ran a few checks and said its a faulty pcm. My neighbor runs a transmissions shop and he also said the same. But neither can tell me if I can use one of another truck without flashing like I would have to with a new one. I have a used pcm on the way. Just want to know if I'm wasting my time. All this stemmed from the green engine cleaner at one of the new self car washes. It corroded my battery post so bad the truck wouldn't start. I ran out of change and didn't wash that crap off.

Input please
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