s10 year model 1996 gearbox not working right
 

Hello.. i have blazer year model 1996, driven to 178 000 miles.... gear box is totaly rebuilded(not me ), turbine are fixed. reason why gearbox have rebuilt was that fourth gear is missed ( lasta owner is pulled trailer and small exvator to big d gear) .Gear box works fine to city drive but when i go to highway and push pedal to medal it´s runing only third gear and hightest speed is only about 50 miles ( 90km/h) and engine rpm is almoust red. when i littlebit gieven up go gas pedal goes fourth gear and turbine lock on.. and running littlebit over 70mph. and engine rpm is about 2000rpm but when is pull litllebit gas it´s changed back to third gear.
in fourht gear engine rpm is about 2000 and when turbine lock goes on it´s 1500rpm in speed 50mph but over that speed not growing

it´s litllebit difficult to drive in that way and bypassing to other cars... the car was stayed to two years (too much work) but i remember that working was difficult before.... i have also in caprice stw to 1986 and i been have older caprice, buick and oldsmobile stw, g20 chevy van etc... and i was been car mechanic and now i fixed tractors, but this vortex ja 4l60e combination is new to me.
i have tested to tps sensor, it´s linear to 0.5-4.4 so is in tehere anyting else what can be broken? or is in gearbox something wrong? this gear slipping?