sounds silly but do you have the correct amount of ATF in the gear box ? as it warms up it will expand and then the correct level is there and it will select reverse but when cold only forward gears will be able to be used failing that i would say the F seal on the box have given up the Go, if the gear boxes are serviced every 20k they are fairly good units, i am yet to report any kind of fault on my box and its now on 151k and its still silky smooth, i do a full oil and filter change every 15-20k and keeps it in tip top condition,
i use this stuff every second box service
http://www.wynnsusa.com/pdf/spec-she..._Flush-Kit.pdf
and then follow normal filling and service
best i can do sorry