From speeds 55mph to upwards, more apparent underload on incline

Lucky car covered by extented warranty & factory hv so far replaced

Alloys
Diff
Tansmission
Prop

 

I bought my car used and I had a similar problem for months. I had Dealer"force balance" the wheels but thatdidn't fix it. Months later I finally needed to replace the tires due to wearand loosing traction in the rain. I put Michelin Pilot Sports (dealer recommended) on myDetriot rims and had them filled with nitrogen. It was like driving a completely different car! Ride was smooth as glass, when the road cooperated. Original tread wear must have been really uneven.