Truck slows way down and I have to downshift back to third gear?

My truck has a recent rebuilt engine and transmission. It runs fine on low inclines and of course down hill. The problem is when we begin to ascend a 4 degree or greater incline I drop the transmission into third gear and increase my speed up to 45 - 50 mph, the rpm gauge is at 4 to 4.5 and I shift back into 4th gear (red line is 5), the truck slows way down and I have to down shift back to third gear.I can go 55mph max at redline. I live in Colorado and have to go over 12,000 ft mountain passes which have up to 7 percent grade for miles. I have to get out of the right lane and cruise behind the tractor trailers and maintain 45 mph if I am lucky. The kicker is if I have to carry or pull a load, the truck really slows down on the steep hills (25 - 35 mph). I did a test drive with the mechanic and he said it is ok to run just below red line for miles in third gear with a load on the truck. I am the original owner and when I purchased this truck in 2003 it traveled up steep hills with no problem with loads; I'm a retired construction worker. The whole reason I put in rebuilt engine a transmission was I figured it was more economic to put, $11,000 into it and avoid going back to monthly truck payments. I have been dealing with this nightmare for 3 years. Am I wrong and should just shut up? What would the problem be?