you might check these guys out. You schedule a date/time, they come to your location and do the job right there. I had them come to my house. 6 hours later, all done. Very happy with the work.
Performance Trailer BrakingAt the time they had 3 install teams that rove different parts of the country. Very responsive. You order/pay for the parts, then after the install you pay for the labor. At the time, earlier this year, it was $2700 job for a tandem axle.