1995 ford windstar

My sons 1995 windstar is downshifting at about 50-55 mph and the rpm’s go from 2000 to about 4000. Any suggestions on what to look for - someone told me something about a module…?

Under what conditions is it downshifting?

How many miles does this have on it?
How has the overall maintenance been?

Generally on an older vehicle when it no longer has the power it needs for a specific condition it’ll have to downshift. If the engine is tired from age or not well maintained it’ll do this more readily, like when going up a modest incline at highway speeds. The RPM jump is normal when downshifting.

When the engine has been running awhile and only at 50-55 mph.
It has 123,000 miles
I purchased the van last year and we have changed the oil every 3000 miles.
I understand the downshifting is normal but it stays at 4000 rpms and is burning a heck of a lot of gas.
The transmission oil looks clean, no burnt smell and the filter is clean.
I took it to a transmission shop yesterday and I want to make sure I know what to tell them to look for - I can’t afford a new transmission!
Thanks! for your time…