Vit D deficiency can cause loads of problems including muscle pain & weakness, bone pain, fatigue, difficulty walking,... so you should definitely get your levels checked. In fact, EVERYONE who reads this forum should!
A lot of MSers have low vit D and many of us take vit D3 supplements (typically 5,000iu a day) as vit D3 is increasingly being found to be important in MS. The level of supplements that we take is way above the levels that GP would prescribe so we buy them ourselves (usually on-line - much cheaper!) A lot of us don't notice any difference because we aren't actually deficient in D3, just low. But it's believed to be protective to have high levels of D3 if you have MS so it's definitely worth doing. MSers who are deficient in D3 should notice a difference when they improve their levels though.
I had my vit D levels first checked in June 2010 after I'd been taking 1,200iu a day for about a month. (The RDA is 400iu - a very out of date recommendation.) My blood test came back as normal, but way below the levels that were recommended by the vitamindcouncil website (the vitamind3uk website wasn't around then I don't think), so I upped my supplements to 5,000iu a day. The next blood test came back as a lot higher, but still below the recommended level so I now take 5,000iu & 10,000iu on alternative days. I haven't noticed any difference in my symptoms, but my MS has been a lot better. I can't say whether the vit D3 has helped my MS or whether my improving MS has masked any effect that the higher vit D levels might have made or whether the vit D3 has done nothing at all, but I would definitely rather be taking it than not. I'm a scientist through and through and the evidence has convinced me that I should keep my D3 levels high.