Originally posted by raidermatt
As others have said, you want to leave other employees out of the equation. If HR and the managers have half a clue, they won't discuss anything to do with other employees' compensation with you anyway.
Bob NC's advice on what to say/write is very good. The format isn't all that important. A business letter format is fine, but so would a simple memo with bullet points. The key is to keep it focused and not let it get too wordy.
As far as who to go to, HR is fine, though normally they won't make decisions about compensation as that is the manager's call.
One question... have you brought up the issue to your manager's manager?
Normally, its not best to go to HR until you have been unable to receive a satisfactory answer from your manager. In this case, since your manager is out, you should probably run it by their manager. Just in general terms... something like "I have questions about my compensation and promotional opportunities. Is that something I should speak to you about since my manager is out?"
They should either answer "yes", or tell you who does have the authority to discuss such things with you.
If that ends up getting you nowhere, then go to HR.
That way, you give management the full opportunity to address the issues, and they won't get blindsided by HR.