When you say the lever works erratically, do you mean specifically the button and light, or the shifting between Drive, Park, Reverse, etc?
The shifting (other than the overdrive off button) is just a mechanical connection through a control cable, I think, for the 4R100, even though I think the transmission itself is electronically controlled. If it's erratic, then the cable needs cleaning or adjustment or replacement. If I remember properly, there is an adjustment at the transmission end of the cable to make sure the lever is in sync with the transmission. I don't know the precise procedure to get it set correctly; I've just seen the adjustment on my motorhome.
