Hmmm, again, I can’t really speak to the hyper/hypo/normal aspect, but it sounds to me like you’re trying to bear down too much when you’re singing – I’d bet anything that you’re overbreathing in general. Excess breath pressure can cause tightness throughout the entire rib cage, and can even mess with your shoulder girdle, where the muscle that controls your tongue is anchored.
A few things to try:
Vocal scans/slides/sirens – start by sliding downward over a relatively small interval (P5 is a good start) towards the middle of your range on your strongest (e.g. most consistent) vowel. Aim for consistency of vocal tone and vowel sound, and AVOID PUSHING, especially as you descend into your lower register. Think about “speaking” the sound more as you descend. You might also consider placing one of your fingers underneath your chin, near the base of the tongue, while you do this – the muscle should remain fluid and should NOT become rigid or hard as you sing.
Once that’s settling in nicely, begin working your next best vowel sound. Over time, begin combining them (this is GREAT for mixed vowel work in languages like German/French etc). You can then begin widening the interval of your scan, and working upwards – I am personally fond of having my students scan up & down whole octaves on “oo”, “ee”, and “ah” once they can scan downward without pressing.
There’s a great book that you might want to consider purchasing for yourself, titled The Diagnosis and Correction of Vocal Faults. There’s an edition available with a CD of examples on it that is VERY useful.