grinding when putting in forward gear?
sounds like your going to need a new transmission (or a reman if available).

when you select forward the control valve sends fluid to the clutch pack assembly and tells it to move the clutches together so they will compress, works the same way in a car, there is one for forward and also for reverse, these drive drums where the discs go have teeth in them, sounds like they are worn out and slipping.

make sure thats where the noise is, the final drive or even the jack shafts on the wheels might be slipping or broken. noise in the trans has a tendency to travel so get a stethoscope for engines and see if you can pinpoint more where the noise is coming from.
Or...
you can just pull the trans and take it apart and see what the problem is.
either way that trans is probably coming out.