1998 2.5 auto. Parked the truck a week ago and it was fine. Went to move it today and no reverse. Fluid is good and it is full. All forward gears work fine and it shifts great but no reverse. Any ideas what could have happened?

 

1998 2.5l Ranger will have the 4R44E automatic

All automatics run on Pressure, pressure holds the bands and activates the clutches.

Reverse requires the Highest Pressure to engage, above 150psi on most.

So you have an internal leak, or your front pump(main pump) is losing pressure.
Loss of reverse is most common sign that trans will need rebuilding sooner than later, although it usually starts off with "slow to engage" reverse before loss of reverse

After sitting, fluid my have drained down thru the leak and you have air trapped in passages, try REVing engine in Park to get fluid circulating, but I am sure you have, thats what most people do when this happens, lol.

You can try an Automatic Transmission Fluid Conditioner, what they do is to swell seals and gaskets to help increase pressure in the passages.

A shop can test pressures on an automatic, there are external test ports, that testing can ID where problem most likely is, but never 100%.