You should have the M5OD-R1 manual transmission in a 2001 Ranger.
Very good trans with few problems, there are 3 rubber plugs at the upper rear that can start to leak, these should be replaced with metal plugs if trans is ever out.
But that won't cause the problem you describe unless trans fluid has leak out.
This transmission use ATF, yes, automatic transmission fluid, NOT gear oil, so make sure that is what you use when you change it.
Is it easy to get into 1st gear when stopped?
If not then clutch issue would be my guess, that effects all shifting, but especially when stopped.
There is also the shifter bushing, read here: TRS Magazine - Shifter Bushing Replacement
If you are going to check this MAKE SURE!!!!!!!, shifter is IN NEUTRAL before pulling it out.
A Ranger going 120mph on a public road would be dangerous, and foolish, lol.
You also have a speed limiter and RPM limiter in the computer.
First look at the drivers door sticker to see what rear axle ratio you have
Read here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
3.73 is average axle ratio
4.10 is for pulling trailers and will have high RPMs at 70mph
3.45 is for fuel economy, lower RPMs at 70mph and limited towing or low end power
4.56 would have a hard time at 70mph, but could tow a tank, lol.
Partially clogged exhaust pipe will limit top speed, you can test for this with a vacuum gauge, or when exhaust is cold bang on Cat and Muffler and listen for insides rattling, collapsed Cat or muffler parts will limit top speed