Is it the same when the engine is off? If so, the transmission has internal issues. Take it to a pro.

First let me say you made a wise choice. These old Rangers are affordable, dependable, and easy to work on.

The manual transmission ones are even better.

I have a 2001 3.0L automatic. Almost 200,000 miles and zero complaints.

So 1st, look under the hood and make sure your hydraulic clutch reservoir has brake fluid in it.

If it has fluid in it, move on.

This sounds like an issue with the shift interlock in the top cover of your transmission.

So to wrap your head around this think about how you shift. 

You move the shifter in 2 directions to select a gear. Side to Side and Front to Back.

Center is neutral.

You pull the shifter towards you and then move it forward to select 1st gear, and then back to select 2nd gear.

Then you move the shifter back to the the point between 1st and 2nd, across (away from you) through the neutral position and forward to select 3rd gear and back to select 4th gear. 

And you do the same motion again only farther away from you and then forward to select 5th gear of back to select Reverse.

When you make that sideways motion you're selecting which Shift Fork Rail you're moving. When you make the forward or reverse motion you're moving that individual shift fork in either a forward or reverse direction. (selecting that gear)

So let's look at all 3 of those shift rails, and the 1st/2nd and 3rd/4th gear shift forks. (The 5th gear / Reverse shift fork is deeper down and behind the rear extension housing of the transmission, you can't see it from here.

CLICK IMAGE TO ENLARGE

Notice the 3 shift rails and how the shift forks move on them.

The 1st/2nd gears shift rail is on the bottom of the picture. The 1st/2nd gear shift fork is attached to it.

The 3rd/4th gears shift rail is in the middle. The 3rd/4th gear shift fork is attached to it.

The 5th gear and reverse shift rail is at the top. Like I said, you can't see that shift fork from here.

Now look at your shift knob.

Notice how the horizontal line represents which of the 3 shift rails that you're selecting.

Notice how the vertical lines represent which way you're moving the shift fork on the shift rail you selected.

Be patient with me I'm getting to the point.

Each of those shift forks have 3 positions. The forward position engages one gear. The back position engages the other gear. And they have a middle Neutral position.

So let's take the 3rd/4th gear shift fork for an example but it holds true for all 3 rails.

Because of the interlock built into the Shift rail selector (this is what your gear shifter's stub is mounted into)

You can't move that 1st/2nd gear shift fork unless the 3rd/4th gear shift fork is in its neutral position.

So you're stuck in 1st gear if the interlock pins aren't working right or the 3rd/4th shift fork is so worn that it isn't centered in the 3rd/ 4th gear "neutral position" and the interlock isn't "releasing" the 1st/2nd gear shift rail to allow movement of that rail.

So like I said, you have a Ranger and this ain't bad. You pull out the seat, remove the floor covering, and remove the floor pan access plate to the top of your transmission.

Remove the 10 bolts on the top cover of the transmission and you have that Top Cover off and you can examine the interlock, the rails, and the 2 shift forks.

Just one more thing you may want to try 1st. There's a stub which your gear shifter is attached to which controls those shift rails. It wears out. It's an easy replacement. You don't have to remove the top cover or the floor pan cover to replace it.

Maybe give it a try 1st. You can buy the kits on ebay for around $40. Even if it isn't the problem, on your 32 year old Ranger, if you wind up pulling the top cover off of the transmission, you'll want to replace this anyway.

So your transmission is a M5OD-R1 transmission.

Plenty of youtubes and internet information on everything about that one.