Unless your 401k is a rare one, your administrator won't let you put into a self directed account. As you say, you'll have to leave the company.
Debbie, you may still be able to move that money out of your new employeers plan. They should track that money as "rollover" money separately from other funds in the account.
I don't think Sterling offers checkbook IRA LLC plans. I have an account with them, and I'm not aware they offer such an option. I ended up moving most of my money out of Sterling because of slow responses on closing out a loan a while back, and into a IRA LLC set up by Guidant Financial.
RV, if you do try to invest in real estate with IRA funds, be sure you're fully aware of the rules. There are ways to trip yourself up and create a prohibited transaction. If you do, the entire IRA is considered immediately distributed, and subject to taxes and penalties. No worse than what you're considering, but if you have all the cash tied up in property, you would be in a hurt to pay the taxes.
I have moved quite a bit of my money out of stocks and into real estate in various forums with IRA funds. However, I still have a 401k, and still have another chunk of change in stocks and bonds. Diversification is key. I'm also very dismayed by the market, and I am not convinced its a good long term investment. I'm prepared to be surprised, but I'd be very surprised if we see the "10% long term gains" in stocks over the next 20 years. Too many boomers who will need money and will be drawing down their savings. Stocks are just another product, in many ways, and subject to supply and demand like anything else. No demand, lots of supply, and prices drop.
Also, do some reading in the Rental Property forum, especially the sticky threads about expenses. Some previous posts by LU members have indicated a, shall we say, optimistic view of rental property expenses. A more realistic view will help you avoid getting over your head.