I don't have access to a compressor small enough to take out with me but i still wanna air down when i go wheelin. how far down can i go before its dangerous on the highway? i've got 31in mudstar tires.

 

i would say anything below 25 is dangerous on the highway.
i wouldn't go any lower than about 17 on the trail. those tires have a pretty thin sidewall.
are these on stock rims?

 

Go to walmart or any auto parts store and buy an air tank pump it up to 100-125 psi and use it to air back up it might not get you all the way to full hwy pressure, but if used sparingly between all 4 tires and you don't air down too far it should get you back up to a safer hwy psi. the problem is volume, you can compress alot of air in those tanks but it takes a lot of volume to air back up another solution is small cigarette lighter pumps, and some jump boxes have air pumps, I personally had an old air conditioning pump of an old Ford truck custom fitted to my engine on an old truck I had and had the outlet plumbed into a tank mounted on the side of my inner fender it wasn't strong enough to run air tools or anything like that but was perfect for airing up, I used this when I lived in Cali. and wheeled daily on Pismo Beach, if you have the money,get the warn winch that has an integrated air compressor I hope to get one hopefully in the near future. hope this helps, keep on wheel'in and keep the rubber side down.

 

Portable compressor with alligator clips works well, around $60.