no overdrive

my grandprix wont shift into 4th. it revs at 3000 rpm at 100 km/hr. if i leave it in drive and let go of the gas at 100km/h the rpms will drop to 1200 but to regain power i have to rev to 3000 rpm. if i shift into 3rd at 100 km/h nothing happens but if i let go of the gas first then shift in to third it reacts normally by reving and slowing down?

Anyone know whats going on?

You stripped the splines off your 4th/OD clutch shaft. Its a common problem on the 4t65e transmission and its 4t65e-hd counterpart. Nothing short of a complete overhaul is going to fix your issue unfortunatly. Replace the 4th/OD clutch shaft with its upgraded hardened part. Its available through sited like ZZP.

But it it was thw shaft wouldn't it wanna try to go into. OD and maybe slip? It dosnt even try which leads me more towards electrical.also arnt 2Nd reverse and 4th all together so should I should lose mmore gears no ?

I will see if I can find a cluth/band application chart.

You could bring it to a trans shop or GM dealer and have it scanned for codes relating to the trans. The shop will need to use a scanner capable of retrieving GM extended service codes P1xxx.