no arguments here, I just like good football talk. and with football talk, you are going to get all kinds of opinions.
I just think that as a true blocking FB, Shelton ranks right up there with some of those that you list. As was the case with Sam Gash (one of the All-Time greatest FB's) they aren't going to ask him to carry or catch the ball that often. Just lower the boom and make an alley for Willis.
Taking your list, I will add a couple and rank them the way I see them, putting an emphasis on blocking and not neccessarily ball handling skills:
Mack Strong (this guy is a big reason Alexander is who he is)
Lorenzo Neil (always been one of my favorite FB's)
William Henderson (GB...this guy is a rock. My favorite FB in the league)
Cory Schlesinger (Det)
Karny
Dan Krieder (I think the rest of the O-line certainly makes his job easier)
Shelton
Terrell Smith (Cleveland...once opened huge holes for McCallister in NO)
Allstot (I used to love MA, but he really hasn't been the same since his neck injury. I also think the Bucs feel he might be on the decline as well because they aggressively went out and signed Sowell from the Jets as insurance)
Now, when i say one of the best in the league, i certainly don't mean top 3 or even 5. With the list that you and i have come up with, there are still 24 other teams, so that would certainly make Shelton one of the best.