Though this is likely still speculation, this Wikipedia page says that Jim Cummings is voicing Grandpa Longneck.
http://en.wikipedia.org/wiki/The_Land_Before_Time_(series)I would still suggest being careful with Wikipedia however-once I saw someone claim that Linda Gary was playing Grandma Longneck in Journey of the Heart on there, so...
However, I also found another page on Retrojunk saying that Jim Cummings would be playing Grandpa. Though this page is at least a couple years old, Grandpa Longneck was the last role on Jim Cummings' long role list, so maybe this page was edited since it was originally posted.
https://www.retrojunk.com/article/show/4015...d-our-childhoodIf this turns out to be true, I'd be very happy! For one, Grandpa would still be in, and two, Jim Cummings has certainly had a versatile career, and I think he did a nice job with Kenneth Mars' Triton too. I'm also impressed with his ability to mimic other voice actors, like when he substituted for Jeremy Irons in The Lion King's "Be Prepared".