Your best bet is to start with your former Attorney. If you did not hire your own attorney at that time, you were probably represented by the Office of the Federal Public Defender in Texas. You should contact that office in Dallas and talk to them.
Now, the ruling you are talking about would help you with a reduction in your sentence, but is not likely to help you with your immigration issues. The problems with the USSG Guidelines being "mandatory" at the time of your conviction really only helps you to change the sentence you received. However, since it sounds like you have already served the sentence, this is not much help to you.
Each case is different and I cannot say what would happen in your case. However, it is likely that your deportation status will not change. Anyway, you should check with the Office of the Federal Public Defender to see if they will take your case, or if they can refer you to a good Dallas Area Attorney.
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