Is my automatic transmission slipping?

I'm new to auto transmissions, been driving manuals for last 18 years.

Just purchased a used '91 F150 with E4OD auto transmission, 105K miles. Also puchased my first automatic car this year.

Going down Interstate on ramp in my F150 get rolling 20 - 25 mph, hit the throttle engine jumps to let say 2,500 rpms and rpm needle just stays there, doesn't move. The trucks gradually increases speed to say

50-60mph and trans will shift, and increase speed some more. But the RPM's will stay at 2,500 until I let off throttle, or speedo pegs out at 85 mph.

I'm puzzled the rpms jump to say 2,500 and just stays there in 2nd,

3rd gear, while the speed increases.

In my car with an auto trans it starts pulling at say 1,200 rpms and rpms gradually climbs proportionately as the speed increases.

Does my truck have a lot higher stall speed torque convertor than my car? Or is the trans slipping in 2nd, 3rd gear?

On a hill in 4th, the speed will start to drop, but the rpms also drop proportionately, which would seem to indicate no slip in 4th gear.

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Eb
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