A Honda with issues

My 2004 Honda Element with just over 105K miles has developed a strong vibration which only occurs under load/acceleration between the speeds of 45 and 65 miles per hour.



My car has a manual transmission and the issue occurs in all relevant gears (3rd, 4th, 5th).



The vibration is most prevalent when getting up to highway speeds or going up hill on the highway. There tends to be less vibration in the higher RPMs and the issue is not present below 40 or above 70 MPH.



I have tried several things but I have seen no improvement. The issue started about 3 months ago and has not gotten significantly better or worse over about 3,000 miles. I would appreciate some advice on what to try next.



I have already:

1) balanced and rotated the tires

2) Changed the spark plugs and checked the wires

3) Changed the PCV valve



Thanks!

Has anyone inspected the motor mounts?

My guess is a bad CV axle.

Is it all wheel drive? Maybe Prop Shaft?