6.9 diesel highway speed problems
 

Hey all.

Just got my first diesel truck. It's an 88 f150 that was originally a gas engine. But was swapped with a 6.9 international with the 4 speed.

Truck has 3.28 gears with 29" tires as well she is lowered. (I will be lifting)

Okay problem time. Truck runs fine in the streets. But on Highway she will only go to 50mph. After that engine still roars but truck won't go anymore. If I push abit more you start seeing smoke blowing out of the exhaust. What could this be? Any help would be appreciated. I'm really stuck.

Trans is slipping