So my 01 IS with 103k sometimes kinda shakes, jerks going up hill at just below 2k RPM's. It doesn't do it all the time. I have noticed that the tach does not seem to drop when this happens. It actually feels like the rear end to me. I had the tranny fluid changed at the Toyota dealership in Sept. Always shifts fine never have a clunking noise or anything like that. What do you guys think this could be? Rear diff? Only ever does it uphill. When it starts doing that I give it a tiny bit more gas and it goes away, even if it don't downshift.

Thanks,
Messy

 

Could be your gas. Try Premium for a couple times. See if it goes away. Also a different gas station.