np231?

Hi I have a np231 question. My friend has a 1990 XJ and It is not shifting into 4lo. we checked all the linkage tried it in diffrent location and it still will not shift. we also removed the linkage and tried to shift manualy. still no go.
Next step is to remove and open it. Any tips on what the prob might be will be welcome.

Thanks

is it that the shifter is going into the 4 lo position and nothing is happening, or that it wont even go into the 4 lo position

try it while rolling forward slowly (2-3mph) in neutral (clutch in if manual)

The shifter will not even go into 4lo position. We rolled it back and fourth in neutral with the motor running still no go.