1999 Dodge Ram

i JUST bought a 1999 Ram 1500 2 wheel drive auto. with 5.2 liter.
the trans worked fine and shifted nicely. drove it for 2 days (aprox 60 miles) then it just wouldn't shift into third. i have tried feathering the gas... manually shifting from (d) 1 then going though the gears but notyhing seems to help. i have disconected the battery cables over night but no help... i have crawed under and inspected the trans and all wires and connectors seem in good shape and connected. have replaced the trans fuse under the hood....
there was no slipping or hard shifting before failure...
i contacted the previous owner who swears up and down that there were no trans problems .
is there a scanning tool that will scan a trans?
i am told it could be a number of things...senors in the trans.... outside the trans...
i guess my question is....
is there a way to fix cheap? i can't afford rebuild, just put everything i had into buying this....
another forum told a guy to get his computer reset... is this a possible fix???
please help..... just don't know where to turn