'88 1-ton TH400 won't get up to speed

[5Tractorguy]

Want to bounce this off some of you...

Just had the TH400 rebuilt in my '88 for the second time. Stock stall heavy duty converter, heavier kit put in the transmission, nothing too special. All the clutches and plates were changed. Not sure what my truck has for diff gears...

Problem is I can't get up to speed. In other words I'm wound out at 45-50mph turning 3000rpm easy.

A few years ago I used to be able to get up to 65mph at 2800-3000 and head down the highway no problem.

My reason for having the trans rebuilt again was the slow decline in my top speed. I was hoping this rebuild would take care of it but something still doesn't seem right.

As a side thought, would a vacuum leak cause the vacuum modulator (which was replaced) not to shift completely into gear?

Anyone experience this before?
Any thoughts/ideas would be greatly appreciated. Thanks!