It might be a simple issue but First I need some questions answered in order to even start to diagnose the problem.
1) What series it the transmission 2000, or 3000 how manny miles on the transmission?
2) When was the spin on filter changed? Do you change it every 50,000 miles or 2,000 service hours at least twice a year regardless of milage I change mine on average 25,000 miles.
3) Was the fluid and filter changed every 150,000 miles, 4,000 service hours, or four years old?
4) What type of fluid you are running Transynd like you should have or did somone get cheap and fill it with Dextron. Dextron is still ok but it dose not perform and protect as good as Transynd.
5) What is the condition of the fluid, cherry red, slightly tinted, brown, black, smell normal or gross burnned smell. Is there bits of metal on the dipstick, how dose the fluid feel clean and smooth or dose it feel gritty,
6) Any new or strange sounds? grinding, banging, moaning, louder than normal operation sounds.
7) Is the transmission slipping or dropping gears?
8) Any burning smell like burned oil miked with hot brake smell?
9) Any codes or trouble lights?
10) How is your engine performance? Engine problems will affect the transmission.
11) Has the transmission locked into limp mode on you stuck in second gear? Or has it shut down completely?
12) Do you still have both electronic as well as manual/hydrolic opperation or has it dropped to manual/hydrolic mode?
13) Is this the original transmission, Allison factory rebuilt, transmission shop rebuilt, or a bone yard special? Yep, theres a difference.
14) Are your cooler fins blocked up with crud, dented or flatened restricting airflow?
15) Are your transmission lines and pan bolts tight?
If you take my advice and attempt any work on your truck YOU ASSUME THE RISK no matter what happens.