So what i am looking to come up as an idea is a tow mode/overdrive off. these modes appear on ford and Chevrolet vehicles and they disable the overdrive in an automatic transmission allowing for safer and more effective engine breaking. the only thing someone would have to do is change the behavior of the automatic transmission when the mode is engaged. Just like a modern column shifted automatic transmission. The mode is usually activated by pressing a button on the end of the column shifter. this mode appears on vehicles like the ford taurus, ford escape, and the Chevrolet silverado.
In older pickups that may have been before the time of the tow/haul/OD off button generally just had 4 forward gear selections. 1,2,D, and if they had a decent tow setup, Overdrive w/lockup. If you wanna haul, you just put it in drive. However, the button generally changes the behavior of the torque converter to deliver tighter and more immediate response with limited slippage. The D series may be before the button, but then it really ought to have 4 forward gear selection options for the automatic trans.
Would like to add that the 4-speed autos in all the IRL versions of the relevant BeamNG cars also have O/D off switches or equivalents. E.g. Covet should have a D3 position below the normal D4. (Going to stop myself now before I go on about how the "2" position should actually work in most older autos)
