One problem I do have is getting the tenderloins out without them shriveling up. This last time was the best I have done so far, but on previous attempts they have always contracted and gotten much shorter than I have ever received from a professional processor. Not sure what's going wrong there.

Shriveling up how so? how are you getting them out? Only way I had one shrivel up was sitting in a fridge and dried up some.

With deer hanging by hind legs and gutted I make a cut along pelvis region to detach the muscle then pretty much just pull down and cut the other end.

Not sure how to describe the shriveling... imagine a rubber band stretched out. Unstretch the rubber band and it becomes shorter... or shriveled up. That is what the tenderloins tend to do. The individual muscle sections contract and shrivel up.

I'm basically doing the same as you, although apparently not. I'll either cut at one end and pull out, or just use my hand to separate the middle and cut either end.

Some shrivel up more than others.

I have never noticed it with tenderloins unless they dry out, but I have seen fish fillets contract like that when frozen and thawed out.