For some reason I've been off and on trying to remember for this for some years now.
In short: The protagonist wakes up to discover his body has been replaced by a tank and is being forced to fight to in a war for his alien enemy against some other enemy.
I read this most likely in the mid/late 1980's. I'm fairly certain it was a paperback and was probably a few years old by then. I think it may have had an outline of a tank on the cover. I'm fairly sure I had read "Armor" by John Steakley in the same general time frame.
More detail: The protagonist was on Earth as part of a resistance against an alien invasion. Near the beginning of the book they had augmented his strength, reflexes, etc. After going through all of that, he was immediately killed in his first encounter. Eventually he wakes up again to discover that he is now a brain in a box and his body is now a tank fighting some other war. He was likely able to break free and regain consciousness because of the augmentations he received. I believe that from there he went about waking up the other human brains and leading a revolt.
Not sure why it stuck in my head but there was a scene when they were testing his augmentations. They had him in a weight machine and he was complaining that the shoulder padding was too soft because the bar was digging in his shoulders. The comment back was that the pads were made out of oak or some such.
Does anyone recall the name of the book, author, etc so I can track it down again?