hi anyone know about a 2000 dodge ram 2500 transmission it doesnt shift right?

[Alla]

hi my question is about my dodge ram i just got its a 2000 2500 series the transmission has a strong shift with no slipping even under pressure but when i first take it from park i cant put it into drive cause it takes off in 3rd gear but from park to first gear it starts in 1st then i can shift up too 2nd and then 3rd but when i come to a stop it wont down shift i have to manually do it so i put it back down in 1st and it takes off in 2nd then i have to shift up to 3rd and it has the over drive button option but 4 gear just don`t work im not sure if someone put a 3 speed tranny in it cause it has a jasper sticker on it but i can only do about 70 on the highway and thats topping out at 3 grand that's all it has the guy who i just bought it from said it has 410 gears and it was set up for towing could he of had a 3 speed trans put in i just don`t get it and i just replaced the output sensor on the trans but it did nothing i hope the tranny isn't bad like i said when you manually shift it it goes in the gears fine no slips or jumps can some one help me with this pleassssseeeeeeee