Vehicle is a 1991 Ford Explorer 4.0 engine and 4wd with automatic transmission. Yesterday, while driving my normal 25 miles to work, I noticed, that my normal rpm of about 2000 at 60 mph was reading about 26-2700 rpm. When I stepped on the gas, vehicle seemed to accelerate OK. At one point on my way to work, I have a fairly steep grade for a mile or two. Vehicle seemed to buck 3 or 4 times close together. My initial theory was that I was low on fluid. Checked dipstick in Park with engine running. OK! Then I thought maybe transmission was slipping in high gear.? Tranny seems to shift through gears positively. I called local transmission shop and he mentioned that there are solenoids and that maybe the transmission may not be shifting all the way into Odrive or locking in. . Couldn't really tell unless I brought it in, which I intend on doing. What did he mean and is this I could repair at home? By, the way, when at 50 mph, I shifted into Drive and it was OK. Shifted back into Odrive and it seemed to shift OK. Is there another gear that I can't feel?

Personally, I would try this first:

. Drop and clean the transmission pan and magnet, change the filter, and refill with Mobil 1 Synthetic ATF. This is likely something you need to do anyway.

. Check for DTCs and pending codes, with an advanced scan tool. If nothing shows up, I doubt the transmission shop will find any concrete evidence of a problem.

. Replace the Vehicle Speed Sensor (VSS). The VSS sends a signal to the on-board computer reporting the vehicle speed. When not replaced, the vehicle may experience CEL illumination, high fuel consumption, loss of power and/or inaccurate transmission shift points. $16.99 at AutoZone. Should be easy to replace. Failing VSSs cause all sorts of problems on older Ford transmissions. The VSS on your Explorer has a mechanical component, which is subject to wear.