8 3/4 pinion shim question

Do you have a tool to set pinion depth? There's really no other way to accurately check it. Without the tool, you're just guessing. Kinda sounds like you may not have one?

If there were no shims in there to begin with then yes, it was probably apart before and assembled incorrectly. There is an outside chance that shims may not have been necessary but that's not likely.

Basically, if you're starting from scratch, you need to measure pinion depth.

And again, there should be a number either marked/scribed etc. on the gears somewhere which should be your target depth.

Let us know how it turns out.