It is a good idea to get the OD unit flil plug loose before you try to change the fluid. Be careful as the OD case is aluminum. The OD fill plug is on the driver's side and is a square head plug next to the OD solenoid connecting wire. Same goes for the fill plug in the 4-speed trans. The 4-speed portion is actually a Borg-Warner Super T-10 and it will use 75W-90 gear oil.
With the tach showing 2400 RPM assuming you are in 4th gear, it sounds like the OD is not working. At 60MPH or so, 4th gear with the OD engaged should be about 1600-1700 RPM. You can easily test the OD operation by depressing the throttle in 4th gear at freeway cruise speeds; the system has an automatic kickdown sort of like an auto trans. If the OD is working, you will feel the "downshift" and once you release the throttle pedal, the unit should go back into OD.
There are a number of things that could cause the OD not to work. There is a solenoid and pressure switch inside the OD unit that is vislble when you drop the pan. Check for loose wires or connectors. The solenoid or the pressure switch may be bad. There is also a OD relay on the firewall next to the windshield wiper motor. That may be bad. It's a common relay to other components (the fuel pump relay is the same part #)
If you like working on cars, I would suggest that you get a real GM Service Manual printed by Helms for your 86. You can find them on eBay or in the parts for sale sections. You want the version with the
RED covers, not the white covers. Red covered books are the production version and the white cover books are the preliminary versions. The manuals are a two volume set so make sure you get both. They are the same books that dealership service techs used. You may also be able to get them directly from Helm: www.helminc.com
For jack stand locations, you can place them under the frame pinch welds. Look at the jack placement points that are molded into the lower rocker panels. USe a floor jack at these points to raise the car. In the front, you can place a jack stand on the small vertical tab on the bottom of the frame just under the battery or about 4" in from where the tab curves inward.
At the rear, just a fww inches inside and in front of the rear wheelwell, you will see the same type of vertical tab wher it forms a square. You can place a jack stand under this area.
The C4 body is stiff enough that you can raise one side of the car at a time even if you jack the car up at the corner jacking points. I place a floor jack midway between the molded jacking points and raise the entire side of my 87 at one time.
You do need to look under the rocker panel to find a small threaded stud about in the middle. On the driver's side, it holds the brake lines in place; on the passenger side, it holds the fuel lines. Just position the pad of the floor jack either just in front or just behind the studs and you will be fine. Be careful to not have the pad of the jack touch the floor pan.
Here's a link to how the 4+3 operates:
http://www.5speeds.com/43table.htm
This link to SKSpeed in NY state is a help if you need parts or tech information.
www.skspeed.com