downshifting problem

between 45 and 55 mph the engine strains like a standard transmission would when you give it gas when you have it in too high a gear. never noticed this before, could this be the start or some transmission problems. thanks.

Vague description, this could be a loss of engine power or delayed shift, from here tough to tell, have someone knowledgeble drive the truck.

I will, thanks for your reply. Maybe a tune up might help. I have 100,000 miles on it and never done one.