quadra-drive lo gear pops out

[cfmcgrath]

I have a 1999 Grand Cherokee Laredo 4wd quadra-drive.
I initially had problems getting the hi-lo shifter into lo because I think it had not been used for 4 years on anything but the normal drive mode.
I wd40'ed the linkage under the car and it moves now. BUT...
After following the instructions I found here about shifting while in neutral but moving 3-5mph I can get the lever into low position but I hear and feel a popping and shaking (like gears grinding but not continuous) when I shift back into drive and hit the gas at all. Needless to say, I shifted back out of drive to neutral and shifted back to high mode. Any ideas where I should start to diagnose this ?