Hi L.,
I, too have psoriasis of the scalp (and guttate p. all over my body) so I can relate--it's a horrible disease. Unfortunately, there is no cure for p., so all you can do is try your best to get into remission.
To help my scalp, I use the T-Gel shampoo you mentioned, plus a solution from the dermatologist--it's called Lidex (or Fluocinonide is the generic term). A little greasy, but I just sleep with an old pillowcase on nights that I use it. Also, I know there are different solutions of T-Gel--the higher the coal tar content, the better. I use the T-Gel Therapeutic with I think 15% or something like that. It says right on the bottle it's for psoriasis. I get mine at Target; it's in a clear bottle and the shampoo looks a little like Hershey's syrup (but doesn't taste as good.:) It doesn't discolor your hair, but after you use it, if you condition, be sure to avoid the conditioner right on your scalp or you deactivate the shampoo.
I am also doing narrow-band UVB treatments through my dermatologist. They're a nuisance, but SOOOO helpful. It's sort of like tanning, but with different light (and you don't really get a tan from it). It has less risk of cancer than tanning, and is done under the care of a dermatologist. The type of light from this treatment helps your body produce Vitamine D, and basically help rebalance your immune system. (P. is an autoimmune disorder, so it can be flared by things that trigger your immune system.) I use an all-body unit, but they have little hand-held models attached to a comb specifically for scalp p. I would highly recommend this treatment because it's not invasive and has little or no side effects.
I would imagine being pregnant is a flare trigger for you, so this may get better after you have the baby. In the meantime, I would stay away from biologics (like Enbrel, Humira, etc.) There are some serious side effects associated with them even if you're not pregnant. My derm is trying everything possible to keep me off them. BUT, they can be extremely effective.
If you haven't already, check out the message boards at the National Psoriasis Foundation--they have been an incredible resource for me and the people are amazing. http://www.psoriasis.org/home/
Best of luck to you...it's an incky condition, so I'm keeping my fingers crossed for you!!
S.