sounds like a pinion angle problem to me...the reason it gets bad/smooths out is because the frequency has increased high enough to be above the resonant requency of the car...at 110, you're between the first and second harmonics.

1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)