1986 Toyota Corolla 5th gear acting up

Transmission problem
1986 Toyota Corolla 4 cyl Two Wheel Drive Manual 1xxxxx miles

I had just recently had the 5spd transmission rebuilt at a local garage and now im having problems with my 5th gear.

The problem is, when im driving at regular speeds( around town) I can shift into 5th gear just fine.

But when I get on the highway and I shift into 5th gear there, the engine revs up high (3500 RPM) and it feels as if I had just shifted down to a lower gear, I gain speed, but I don't think my transmission is supposed to do that. Do you perhaps know the cause? Am I just not shifting at the right speed?

The Shift Speed stated in the owner manual sets the Shift-Up from 4th-5th at 45MPH and that's where I have been shifting.