The prior owner had fluid and filter changed about 34k miles ago. He had a file of service records he gave me when i bought it. That was about 1 1/2 years ago.

 

When was the tranny last serviced?

 

With the mileage on your truck you should be doing a fluid change every 20K to get as many miles as you can out of it.
I would do a drain and refill and add a bottle of Lucas tranny additive to help free up the internal valves and see what happens.