Hello all, I have a 98 Ranger 4.0 4x4 with an automatic transmission. When I shift into Drive or Reverse it goes nowhere and engine revs. But if I shift into 1 gear it drives and manually into 2 gear at about 10mph it shifts and it drives with no problem. If I go down a steep hill and get up to about 40 mph and shift it into Drive it will shift into OD and drives fine until I slow down then it acts like it's in neutral again.
where do I start troubleshooting?

 

Welcome to the forum

1998 Ranger 4.0l will have a 5R55E automatic transmission

The Forward one-way clutch(sprag) is bad based on your description, which means a Rebuild, its way inside the rotating part of the transmission
The sprag(one-way clutch) is used to engage D or R, but if you shift to 1st manually the Coast clutch will get you rolling and then computer can start to shift gears, which is why it seems "normal" once you are up to speed.
Won't last long that way, lol, and you will need to flush out the trans cooler and lines because there will be metal particles in the fluid now

If you look for used transmission.............
Because you have a 1998-2000 4.0l Ranger you can use a 1998-2011 5R55E from a 4.0l 4x4 Ranger or Mazda B-4000 4x4
Must be from 4x4 and 4.0l(OHV or SOHC doesn't matter)
2001 and up have 2 extra sensors that your computer doesn't need, so they are just not hooked up
The 16 pin wiring plug is exactly the same
These years are plug and play for your 1998