Haveing problem with my 1998 24v dully.My tran temp light come on and OD too.It will not shift into OD.At 60mp it running about 2500 rpm . No mods .Flud fine

 

chack and see if some how the connector on the tranny came unplugged
good luck with the problem man

 

Might want to put a code reader on there and see if you have any codes that are in the system. That may help you track the cause down quicker. Also, I start thinking something electrical, check and clean battery terminals and grounds.