Can I legally tell my 17 year to find somewhere else to live?

Generally, NO. However if there are serious problems with drugs, alcohol or something else, get the proper authorities involved.

You can get him legally emancipated in many, if not all States - I've seen people have their children emancipated for little or no reason.

What is the problem here?

Could we get some more info and what the problem(s) is with the child?

The cliff notes version...
Got him back from his dad Oct 07 after almost 3 years with a daily pot smoking habit. This lead to quitting football, getting kicked out of all county schools, being place under the jurisdiction of the county juvenile judge, multiple failed drug screens, misc short sanctions in the county jail (one as long as 20 days), 3 arrests, 70 days in residential rehab from which he was kicked out twice, most recently 60 days in jail for contempt of the juvenile program and soon to go to court on all the charges for the arrests... 3 felonies for anger related charges and misc misdemeaners. Got out of jail one week ago today.
I've offered another rehab/recovery program. He went one day last week then back to the same old routine of "hanging out" and getting high. . Turns 18 in 3 weeks, but I want to tell him now to "get help" or "get out".
What do you think?

Jesus this boy is crying out for some help. Either that or he really doesn't get the slyly doing ILLEGAL things?

I think I understand where you are coming from and that you feel you need something drastic but, and I'm guessing this isn't just about getting him out it's about helping him too, please advise me if I am wrong.

Why not try something a little different. Since you and the state have tried the heavy handed thing and he doesn't seem to be getting it.

Here's a few ideas:

Try to get access to a adult rehab center and show him the Class A drug addicts.

Depending on where you are, there is in most big cities in the US, a shelter for addicted women with babies. Sometimes a SCU in the hospital.

What I am saying is try to show him the human aftermath of drug taking.