Unknown Shaking/Vibration

[blue thunder]

I thought I had experienced most every symptom in my car in over seven years but this is different. The past few days about half way to work the car starts to literally shake pretty bad. I can feel it all thru the steering wheel. This starts at speeds above fifty and closer to sixty. It feels like its in the drivetrain somewhere but don't know for sure. I drove home tonight and did not exceed fifty and it never happened. I have new tires for around two months balanced correctly and a good alignment. BTW this doesn't happen with the brakes on. I have slotted and drilled and never get that anymore. I'm worried my front differential could be going. I got a code 11 and 37 and need to address this as well. Any ideas/suggestions greatly appreciated.

Mike