95 jimmy tranny problem

I have a 95 jimmy that at low speeds works fine shifting, but when i go out on the main road and get up around 70 Kms it seems like passing gear is slipping because the rpms shoot way high until I let off on the gas, then it kicks in again,but as soon as i use it again it does the same thing, but any lower speeds its fine, could passing gear be gone or a bad torque converter be at fault, any help would be great,thanks