96 GM 4x4 rough reverse

For some reason I have noticed a problem on my truck, while shifting into reverse. The reverse gear makes the truck vibrate (only when pressing down on the gas pedal), almost like when your driving a stick & your holding the clutch half open. Motor revs but your getting little power.
All the drive gears work fine, only the reverse is rough..
The engine itself is in good condition nearing 700000 Clicks, no engine light warning.. The stock transmission was changed already changed for a new tranny with low mileage. (4 or 5 yrs ago) All the mileage is mostly all highway.
What could cause the reverse to run rough?
Thanks

The symtoms are showing internal problems with the reverse input clutch, lo/reverse clutch, valve body stuck valves or oil pump assembly. Good luck

I forgot to mention its automatic.