"Is it possible to roll an active 401(k) balance to a self directed IRA without leaving my employer?"

Yes it is but it you can't rollover the whole thing as I found out recently as I am below 59 years old. It is quite convoluted. Call your plan sponsor to get the two documents you are required to read before they will let you do it.

It's not termed a rollover but a withdrawal and there are several different types of withdrawals. I made a non-hardship w/o suspension withdrawal to get some money into my traditional IRA. There are several categories your 401(k) money will fall into and you must make withdrawals from the categories successively. You also cannot withdraw any money which has been in the plan for less than two years.

Like I said, read the documents. It's very convoluted. I was disappointed to find that I could only withdraw significantly less than the total amount in my 401(k) plan without penalty.