I'd still try to do testing
So long as you have a user base you can still do testing. Just go to the existing users in person, with a bit of paper with the field on it, and ask them to write down what they'd write if they saw that on a sign-up form.
I would have started with the hypothesis that the problem here is the word "Full", which is keying your ...
Does your system allow users to control their own display name at a later point in time? E.g., once hired, can Robert V. Jones go into some settings panel somewhere and change their own display name to show "Rob Jones" or "R.V. Jones"? (If not, what happens if they change their name legally and want to be subsequently addressed by "R.V. Smith"?)
If so, I'd ...
Display name is just alright.
With this wording, nearly all the managers that try to register new people are providing answers that are clearly intended for usernames, such as bjones, either ignoring or completely misunderstanding the help text above the field.
Just show ask for "display name (example: Robert V. Jones)" and "account name (example: bjones)...
This is a great question and it may take a few more attempts to get it right. As a rule, people don't read, so a short description is key. I'd try:
Preferred Full Name
Nicknames are fine, titles are discouraged
Hope this helps
You could check Google search volumes for those phrases. It is also perfect case for easy A/B test.
Most precise would be "Short URL", as "link" is broader term, and "tiny" is not only related to length. But to test is the best. :)