The 4R100 does not shift when I want it to. It always seems to be in a higher gear than I want it to be.

For example, if I'm on the highway cruising at 70 MPH/2100 RPM and the road goes from flat to slightly uphill, I can add a bit of throttle and the engine will get louder, but I'll still lose momentum.
If I add more throttle, it'll downshift and accelerate. But if I just tap the "Overdrive" button to turn it off, the truck will stay nicely at 70...

Of course for additional engine braking on downhills, I downshift the transmission using the "Overdrive" button and occasionally with the lever.

Is there any sensor or cable that needs adjustment?

 

The shift strategy can be adjusted with a good custom tune, in my opinion the better trans operation is the best part of my custom tunes and really improves the towing ability of the EX.
Maybe it's your situation description, or your view of exactly what "slightly uphill" actually means (Local terrain tends to define what folks call hills and mountains) but if your unloaded EX isn't able to maintain 70 MPH on a slight hill, I think that something needs attention. On my EX running the 89 Performance tune it can typically hold 70-ish MPH transitioning from the flats to slight uphill with up to 60/64% throttle opening while holding OD. If the hill is steeper or longer or I open the throttle more it will drop down to third gear and at that point it can accelerate pretty easily, often I'll let it get back up to cruising speed and let off a hair to allow it to shift back into OD. All of this is while towing the nearly 12K TT and mine seems to have enough juice at 60% throttle to handle most Interstate Highway grades in OD (sometimes in-out-back into OD) excluding actual mountains here in the East.
It sounds like you have the more common 3.73 gears, what size tires are you running?

 

It can maintain 70 MPH with additional throttle. What I'm saying is that I need to apply less throttle if I downshift manually.

Also, someone installed a 2.5 inch exhaust and catalytic converter, so I go from the stock manifolds and Y-pipe, which are 3 inch if I recall correctly, down to 2.5 inch, to the catalytic converter... Then into a 4 inch muffler and tailpipe.

I found that the exhaust from the catalytic converter on back to the tailpipe was welded together when I tried to install my Hellwig Sway bar. It was all 2.5 inch and hit the sway bar when I was installing it, so I cut some of it off and installed a 4 inch catback meant for a 6.0L on my truck.

I have 3.73 gears. I am running 275/65R20 tires (34.5 inch diameter) on 2014 Platinum stock wheels.

 

3” larger tires are definitely NOT helping you

 

You have a pretty unique exhaust system there..........Not sure how or if that would affect this situation though.

Those taller tires are giving you an effective gear ratio of 3.42, my setup has an effective 4.39 ratio. That much of a higher ratio is a big part of how your EX and mine perform differently on a hill, my tunes and how they change the trans shift strategy just adds to that difference. I suppose that tall of a gear ratio would take away a lot of the climbing ability of the 6.8 in OD as it makes better power/torque at higher RPMs than you are seeing in OD.

 

The catalytic converter is also probably shot. It's an aftermarket unit. Those never last long.
I bought the 4 inch exhaust because it was cheap and I never really intended to keep the V10 long. Too many cylinders... WAY too many spark plugs.

 

CORRECTION:

Okay, I just took the Excursion on the Pennsylvania Turnpike because that's the only place I know of where the hills are marked on % Grade.
I had to downshift out of Overdrive to get up anything marked 5% grade or larger and maintain speed.

Also, apparently, I am not used to doing 70 MPH indicated. I'm used to doing just shy of 80 because in Virginia where I live 80 MPH is considered "Reckless Driving" and it moves from a simple speeding ticket to criminal prosecution.

Also, It occurs to me that due to the difference in tires, 80 MPH is not really 80 MPH.

 

Your tires are too large for the gears. I had the same problem on an older truck. You should also use a gps to verify your speed, 80 indicated is probably 85-87mph

 

You need to swap gears to at least 4.56. I would recommend 4.88! And after the gear swap then I would get custom tunes from 5star tuning. They do an excellent job of getting the shift strategy correct.