After I left my employer I rolled my 401k over to a traditional and Roth IRA (Roth for after tax contributions, traditional for everything else), but am having second thoughts. Is there any way to "undo" the rollover(s)?
Probably not. You've left your employer, so you aren't eligible to contribute to the 401(k) anymore. After you've left, they are required to keep your account open as long as you like, but after you've rolled it over to something else and closed your account, they are not going to let you open your 401(k) account up again.
However, you need to ask yourself: Why do you want to go back to your 401(k) from your IRA? 401(k) accounts are generally very restrictive as to what you can invest in; they usually give you a small list of investing options. IRAs, on the other hand, are generally open to any investment you like, limited only by your imagination. If you aren't happy with your current IRA custodian, roll it over into a new one. I'm sure you'll be able to find an IRA with the same investment that you are looking for from your 401(k).
It is unlikely that you can undo the rollover and put money back into the 401(k) plan sponsored by your ex-employer, and even more so if your departure was acrimonious. You could, of course, always ask your ex-employer for permission to put the money back, but I suspect that a rollover from an IRA into a 401(k) is not permitted unless you resume employment with your ex-employer.
The real question is what is it in your ex-employer's 401(k) plan that makes you want to go back there? As a general rule, 401(k) plans have much larger expenses, and a lousy selection of funds in which to invest.