Diesel Bombers - 82 international slammed to stop

hello yall, im trying to use the guideline as much as possible. but here is the problem. i was just driving home and came around one of the last corners and the truck slammed to a stop and refuses to go forward. i tried to shift it into nuetral and it goes into reverse and the reverse also works. i used the entire 4 lane intersection on the highway and was able to get it turned around and drove two miles home in reverse.
Year of truck: 1982 International s1824

Motor: dt466 automatic 6sp

Mods: the only mod is on the fuel pump

How is the truck primarily used: i use it for towing

When did the issue start, and if it's intermittent or all the time: first time

Anything else that might pertain to the issues at hand: prior to this problem it was difficult getting it into first gear. i believe it is due the clearance on the shift lever.

Any codes, and what are you reading codes with: no codes too old

Other: the only thing i can think that might cause this is something in transmission, or the torque converter...MAYBE. or maybe the tranny fluid level but those are all stretches. as soon as i post this im going to check the fluid level but please anyone any information would help. if theres any questions ill answer as soon as i can. thank you in advance.

Robert