1984 Bronco issues

I have a 84 Ford Bronco with the 4.9 liter 6 clylinder and a 4 speed.It runs like a champ. My problem is this. I start out in 2nd gear it will accelerate fine at first, after a few seconds in gear it will bog down, then i shift into 3rd, same thing. then 4th comes along and its all bogged down until i reach about 45 -50 then it seems like it releases and it drives just fine. after the release at 45-50 it doesn't bog down at all through any of the gears until i actually shut the truck off. as soon as i shut the truck off it starts to bog down again until i get to that certain point. anybody have any ideas?

Is this a cold start problem that goes away when it gets up to operating temperature. Or will it do this if starting a warm engine also.

It still does it when the engine is warm but not as bad.

If you don't have any computer codes I would suspect a dirty carburetor.
Kits used to be cheap but when they added a couple of electrical parts they tripled the kit price even though it didn't have these parts. You could try a couple of bottles of carb. cleaner in the gas tank. Generally if your careful and test drive you can feel the difference between a fuel problem and an ignition problem.

Check the vacuum motor that controls the choke pull-off mechanism.
If it does not hold vacuum and slowly leaks it will allow the choke blade to partially close after a few seconds of vehicle acceleration and will produce exactly the symptoms you describe here.

A new choke pull-off vacuum motor is quite easy to install.