A25c won't accelerate

Hi all new to the forum here and looking for info on an A25C. The problem I'm having is that when you put the truck in gear it won't really rev up and go anywhere. If you just move it around at idle it will move fine, but if you try to accelerate it just bogs the motor down and has no power.
The Motor was rebuilt when we got the truck and seems to run good, other than revieg up a bit slow from idle. This kind of leads me to believe it's something with the trans, like possibly the retarder...

A25c won't accelerate