Hello, Dave here, contributing to RepairPal and providing expert advice since 2010.
I suggest proceeding with caution. I personally feel skeptical when I hear "it needs a computer". In my 30 years doing this stuff, I can tell you that a new processor is the solution a very, very small percentage of the time. I'd say less than 1 percent.
Now, I don't know your car and can't inspect it personally. We trust our technicians near us to be professionals and perform proper testing. This is why we offer RepairPal Certified shops -- because they pass a certain standard to be the "best of the best" near you. Historically those shops stand up and have proficiency and can be counted on. So, I encourage you to get a second opinion from a shop as listed below.
Regarding your concern, you do have shift solenoids that do the job of shifting the gears as commanded by the various inputs to the engine's processor. But I'd find it super hard to believe that some kind of computer control issue is causing the problem. The processor in a 2005 Avalanche is not making any "decisions" -- it's feeding off of input information to allow other functions to occur as a result based on what is "correct" for operation. My guess is: you have commands that are not resolved successfully in the transmission until you "force" the issue by putting into reverse and then drive again.
A diagnostic tool can be connected to this vehicle to actively check and command transmission controls. If a command is given and the transmission and shift solenoids respond -- then the mechanicals of the transmission are OK as well as the internal shift controls electronically. If it's commanded and does not respond properly, then you could have an internal transmission concern.
My sense is that you need a reliable diagnostic test performed. And I will continue to bet on the fact you probably do not need a processor.
https://repairpal.com/auto-repair-near-me
Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.
