Hey Jack - welcome to the TDR. Is your truck a 4 X 4? Need to add info to your profile more we have the better we can help in a pinch.
I'm assuming you smoothed down and bumps with a file and greased the flat mating surfaces and bolt/pins when you installed the calipers? I agree with the others with checking your rear breaks.
When I first bought mine it pulled like crazy too - I learned what to do right here on the good ol' TDR. I had to replace calipers, r/rear axel seal and rear shoes to get it to stop straight. To keep it stopping straight I need to regrease the calipers every so often. If there was a spacer between the wheel and the rotor, and you took it off - try replacing it on the same side it was removed. Good luck.