2000 Ford explorer

OHD light flashing staying in third gear with rpm 2500-3000. Two transmission shops say transmission needs to be replaced. Has 78000 miles on it. Bought 6 months ago from original owner. Repairs estimated $1800. From reseaching the problem I see where others have replaced transmission only to have the problem back 6 months later. Any input from other Ford owners regarding this problem would be appreciated.