So as the title says my ranger wont go past 50mph unless its down a long steep hill. Recently I have replaced the plugs, wires, air filter, both fuel filters and the fuel pump inside the tank. She gets up to speed and has seemingly plenty of power in 1st, 2nd and 3rd gear. But when it gets to around 40~45mph and I shift into 4th it usually just bogs down and rarely makes it past 50mph.

My ranger is a 1988 XLT 2.9L V6 manual trans 4x4 w/ manual lockouts.

Im thinking its either in the fuel delivery system or in the motor itself. Ive never dealt with fuel injection before, how do I determine if the injectors are suspect? Anybody know what kind of compression I should expect from the cylinders?