Usually you can feel the transmission shift smoothly in the overdrive mode when you speed up.You have to really get the speed and RPM up to get the transmission to shift into the proper mode. The engine also pings at 40 MPH ,but the pinging goes away at lower RPM or above 50 MPH. Is it possibly a timing issue?
Auto Trans does not shift smoothly on acceleration. Engine also pings @40MPH on 1994 Toyota Pickup
I would have the trans repaired first. it may be causing the motor to lug and ping. have a trans shop check it out. it may be as simple as a throttle valve adjustment or it may be internal.