won't down shift

[dbxtr3]

I got a 1995 chevy 3500 2wd auto 16 ft. stake bed truck. The truck will accelerate pretty fast to 55 mph then it is a slug from 55 to 75mph. If I acclerate to 75mph or so the first hill I hit I start to lose speed and it keeps losing speed till it gets to about 50mph before it will down shift.

It has a jasper reman tranny w/ that is about 5 years old but has less than 20k on it.

What is keeping this truck from downshifting at highway speeds? (I have no passing gear)

[trukdoc]

...a 4L80E you're locked in o/d at @58-63mph...has to drop below 60 before it will drop out of o/d{passin gear}...at least that's how both mine are an elect.pump 94 and a mech. pump 93...