Transmission failing?

2001 Dodge Ram 2500
I back into my garage port because there is a grassy slope to the road
I gradually noticed after the truck has set for 24 hrs or more that it was taking more rpm/gas to start rolling or break inertia from a dead stop.

The dramatic evidence of a problem follows

After 2+ hrs of driving to a cabin site where I need 4x4 to climb a steep muddy grade, it pouring out I pull off the tarmac and hit the pedal. I go about 75 feet, and can't go any further up. I roll back down and try again, topping 2500 rpm I can't make it. I back down to the tarmac and put it in D. It moves but at as I said take more pedal THAN, putting it in reverse. I turn around and back all the way up the 1/4 mile to my cabin twist and turns and all. Next day I head down to the tarmac to deliver some torn off corrugated tin roofing from a previous storm to a old couple for a chicken coop. I turn around and no problem I head right up the hill in D/drive.

I suspect something to do with the heat of the transmission fluid is maybe a factor.

During the hill side episode I didn't think to put it in D2 or D1. Testing latter on a flat surface breaking inertia is better, maybe normal in D1 and maybe so in D2.

The truck shifts normally. It goes right along at hwy speed. Lugging along at 60 sucking 20 mpg.

I've scheduled to have the transmission flushed which will replace the filter at the same time.

BUT is that going to fix my problem and is it worth the $400+