While accelerating onto highway and reaching 60mph the accelerator needle increases up to 90mph but the accelerator stays the same

Accelerate while on the "on ramp" until you reach the speed limit + posted on the highway... this way when you reach the highway you will be going the same speed(or close) as the other vehicles ..you can blend in easily.. and NOT cause an accident by going too slow!! On the way down the "on ramp".. start looking at the traffic coming on the highway to find an opening to ease into safely. Use your mirrors and BLINKER... (its easier to slow down a little than speed up)

check the speedo sensor or VSS sensor or wiring for the systems

check fuel pressure and change TPS (throttle position sensor)

Could be a transmission problem or something as simple as bad spark plugs.
The engine is under the most load when RPM is low and you are trying to accelerate, such as when it upshifts.
You should have the computer checked for trouble codes and hopefully that will point you in the right direction to test.
If the spark plugs have more than 50,000 miles of wear you might start with replacing them.

Could be fuel pressure or a blocked fuel filter. need more test information,

Chances are you need another speedometer, or send that one to a shop to have it fixed.

sounds like a fuel filter(cheap and easy) or a partial melted catalyic converter(expensive)

This is a rare one and can be tricky. Since your rpm gauge rises back up when you increase your acceleration slightly but you feel no jerking of acceleration then this tells me that a speed/rpm senser either has a bad connecton or the sensor itself is going bad when the engine reaches an optimum heat while driveing on the highway. Since you feel no acceleration when you slightly press the accelerator down but yet the rpm gauge is increasing back to normal, the throttle body sensor may be compensating for the failed speed sensor/failed sensor connector. Please get your uplander diagnosed properly first to confim this.

have u replaced the fuel filter to see if this helps? that is what I would do 1st.

try fuel injector cleaner in your next fuel fill. sounds lke your injectors are dirty