4.8 transmission problem

[dustie.pilot@gmail.com]

Hi all. I have a 2005 4.8 being a 5 speed auto. Just over 200k on clock. Started having problems with staying in same gear and not charging about a month ago. Dropped trans oil and filter, looked quite clean, but replaced. Also put in trans treatment at same time. Ran good for about 1 month, now seems to be locked in 4th gear. Transmission has never before ever given any signs of trouble. I suspect electrical, any idears