This was the first sign that my tranny was about to lose 4th. When really getting into it to get onto the interstate right around 48-55 mph is where I really had trouble. If I backed off and then re-started my acceleration I could work past the problem "speed." About a week later I lost 4th all together. 1-3 worked without any problems and I could run right up to 68 mph until the truck went searching for a non-existant 4th gear.
Has your O/D light flashed at all?