99 rover will not reverse
 

hello all !

here's the issue at hand:
99 range rover 4.6 hse
reverses at a snail's pace, if at all (feels and sounds more like it's in neutral)
showing code 20 "gear ratio monitoring (4th gear)
also showing code 5 "lock up clutch monitoring"

all else fine.
had just passed state inspection two days prior to the event.

mechanic said it was a transmission problem and referred me to someone else.
i'm wondering if it's the xyz switch and cable alignment more than anything else.

or, could it be the transmission ECU ?

the car has 105K miles on it and has always been impeccably maintained.

thoughts please ?

many thanks in advance.