More than 1 transmission?

I have a 1999 Dodge Durango with almost 140,000 miles on it. We replaced the transmission at 77,000 miles. Right now when it’s in drive, it seems like there is a power failure as you have to push the accelerator all the way down to get it to go. It is fine in Drive 1 and 2 and reverse.

Any ideas on what the problem could be?

PS I’ve replaced almost everything except the engine at this point in time. It still runs great and I’d like to keep it going.

In terms of your transmission, there is no difference between manually shifting the gears or letting the automatic transmission do it for you. You can do either, that’s part of the reason you have a way of shifting gears manually.
IF that works, if you can shift into every gear manually, but the car won’t do it by itself, it seems that you have a vacuum leak, since vacuum controls the automatic end of shifting. Have that looked at and fixed, or you can also just pretend you have a standard transmission minus the clutch and use the manual shift to control rpm. You can do that; it won’t hurt anything unless you are a very bad judge of when you should shift.

What about the higher gears? Are they working?