2004 Nissan Maxima Cause of Steering Wheel Shaking

What would cause my steering wheel to shake when car hits 50---gets worse when I hit 65---

have had it in 3 times to dealer---they say they did front end alignment---just had to replace right front axle under warranty--- have had it 4 month and it has done this since I took delivery.

tech yesterday said it wasn't front end--- doesn' think he can correct it--- also the steering wheel position when going straight is down to the left about 15 degrees.

I always thought this was caused by front end alignment.

need assistance---- thank you very much.
lee