ran fine went to start in morning and wont start i can hear fuel pump for a sec when try to start and the code i get is 12 and 55

from a van in the junk yard and replace the passenger side rear quarter glass in my 91 Grand Voyager SE?

Ok guys, I have a 1991 Plymouth Grand and I drove it on the hwy today and she ran fine like always. I got home and now had to run downtown, as soon as I took off she started shacking real bad. I thought I had a flat, but no flat, I checked all tires and they r good. I don't know if its a brake locked up, or a bearing??? Once you hit 50-55mph it goes away. I have kids so, I don't know if its safe 4 them 2 be in the van or not? Any help would be great! Thanks.