won't downshift above 45mph....is this normal?

[Jason71k10]

I have a 350/th350 combo in my k-10 with pretty much stock engine, except exhaust. If i'm going 40-45 mph on flat ground, it gears down to 2nd, no problem, but any faster than that or if i'm going up hill, it won't gear down, even though the rpm's aren't that high.
Is this normal?? i'm kinda new to automatics, so i'm not sure.
The tranny was rebuilt about 10k miles ago. Thanks for any hepl guys.

Jason