I'm assuming that the tub and shower are separate fixtures:

You're not going to like my answer: It sounds like the shower valve itself is the source of restriction. Judging by your description, you have eliminated just about everything else.

Other than that, it may be possible that the hot pipe leading to the shower faucet may be bent/buckled behind the wall - but this would only be true if it a) has always been weak, and b) you had a really lousy, doesn't give-a-hoot plumber install the pipe, because this option cannot be an accident. I would therefore consider it unlikely.

- John Scates

: In my shower, if I have it on cool water, it comes out in a powerful blast, but if I turn it to hot, it seems to barely have any pressure at all. The water doesn't seem to get that hot either. But in the tub, and kitchen sink, the water gets very hot....What would cause this difference? How can I get more pressure of hot water in the shower. It has nothing to do with the shower head, I took it totally off and it still does the same. A blast with cool water, and weak with hot....