1999 Ford Ranger XLT 3.0L 4X4 automatic transmission

So this is an odd one and in trying to figure out where to start at or if it is two problems happening. I'll accelerate onto the interstate (80mph) and get going for several miles and when I end up having to slow down and try to accelerate again I'll get nothing and have to turn my hazards in and coast to the side of the road. I can put it into park and sometimes neutral and the transmission will engage again. I just made a 700 mile trip and was having the issue for the first 50 miles until I stopped at a shop for advice and they were unsure unless they could have the vehicle for a couple days.

I started going again and didn't have an issue until 300 miles later when it happened again. I pulled to the side and when I tried to put it in fear it was just grinding. I shut the motor off and started it again in neutral. It was still grinding so I shifted it into 4X4 high and back to 4X2 high and it went into gear and I haven't had an issue with it since I arrived at my destination.

The previous owner said he had the issue before and he shut the truck off for several minutes and it started working fine and would only happen occasionally.

 

Welcome to the forum

The grinding noise would point to transfer case, automatics don't have "gears" to grind, they can of course make grinding noise because they do have rotating shafts, lol, but not something that would start working again.

The transfer case has an electric shift motor to select 2WD, 4hi and 4low, and an electric clutch to allow "Shift on the fly" from 2wd to 4hi and back while moving

In 1999 Ranger the 4WD is controlled by the GEM(generic electronic module), this also controls all the electrics in the cab, i.e. lights, wipers, door locks, ect...............
Its the Body Computer

Here is what I would do first since 4WD is not always needed this time of year
Select 4hi then 2WD to make sure transfer case is indeed in 2WD
Then get under the truck and disconnect shift motor AND electric clutch, electric clutch is a Brown wire, it should be on the connector with the shift motor wires but......
On some years it often didn't have a connector, it was hard wired in, so you may need to CUT this wire, if so then do it, but cut it so you can reconnected it easily, splice it.
Put a couple of baggies over open connectors to prevent water intrusion while they are disconnected

Good look here at the shift motor: How to: Rebuild 4X4 Shift Motor | Ford Explorer and Ford Ranger Forums - Serious Explorations

It is possible the shift motors gears are stripped and transfer case is shifting on its own, but it is more likely that a Relay is shorting causing shift motor to move or Clutch to be engaged
In above link's pictures you can see the shift points for the transfer case shift rod, so you could pull the shift motor off to manually select 2WD

Drive truck for as long as you think necessary to decide it is indeed shift control that was causing the problem.
If problem comes back with no shift control connected then it is most likely an internal transfer case issue

Wiring diagram for shift control here