1999 dodge ram 2500 5.9 360

When I say louder it started acouple weeks ago and I thought it was a exhaust leak, but it's a different sound now., and up until today no shifting issues what so ever but when I limped it home I did try to run it through the gears upshifting and downshifting but it didn't respond but i could only get the truck up to about 20mph
As for fuel the truck is a pig that only gets about 13mpg. And I haven't noticed any difference in mileage
Aaron