Fluctuating speedometer

Hello. I own a '97 F-150 with 165000 km. It has a 4.6L V8, 4x4. My question is this:
A couple days ago, my speedometer started, for a lack of a better term, bouncing about 10 kph. So basically, when the gas is applied, or under load, the speedometer will read 100kph. When I take my foot off the gas, the speedometer will increase to read 110 kph before the deceleration of the truck is read, then back up about 10kph when I apply the gas again. It only started doing this over the last couple of days. I also noticed that if I put the truck into 4x4 mode, the speedometer reads true, with no fluctuation between a loaded and unloaded scenario.
Another thing, the OD light starts flashing after driving for a short time. This condition has been going on for a long time, and was told not to worry about it as long as the transmission will go in and out of OD, which it did, and still does.

Thank you. Donation is coming, but in a hotel away from home and have no secure connection, really.