I agree with Dan, that the 4.025 spring is your issue, or possibly the pins that engage the shift lever are worn and pop out of the holes in the shift lever. Or maybe the shift lever itself has worn holes.
I have had my shifter apart and putting that spring and the pins in place is pretty fiddly, so you may not have gotten it engaged properly.
On my truck, the threads in side the knuckle that attaches to the column were stripped, and the shifter worked, but was really loosey-goosey.
