Oh, I have a problem similiar to this, except it's only my legs, not my entire body.
Let's say I'm dreaming that I'm being chased by someone, the bad guy. I can jump, walk, spin, etc. normally, but if I try to run, my legs feel extremely heavy and I don't go very fast at all.
If I was to describe the feeling, it's like trying to run while your waist deep in water.
This is like the most horrible thing.
I used to get it a lot when I was younger, but more like my limbs locking up, especially if my arms were outstretched or behind my back. also happened when running: legs locking up.
Never get locking up now, but occasionally do get the lead-like legs or body.
During your waking hours, constantly reinforce the idea within yourself that you're able to stand up straight and look up. Hopefully you'll repeat it enough that you're convinced of it even down to the subconscious level. The thing that you expect to happen in a dream is the thing that will happen. You're currently assuming that the same curly thing will happen again, so you're trying to replace that expectation with a different one.
Oh, and I have this problem too. I think in real life you're trying to move your legs, but the blankets block your legs, leading to your mind making sense of it that way.
Oh, I've always figured it was probably something like that, seeing as how nothing makes me happier than burying myself under lots of blankets.
This happens to my brother, only in his dreams, cinder blocks physically materialize tied around his feet.