That sounds about right ,first, the rear differential carrier inside the rear housing usually gets replaced when going to a 4:10 gear as the case cannot accomadate that ratio .Secondly the front differential takes a substantial amount of work to remove the main section as it is two halves put together
with the axle tubes bolted to it ,not to mention the end drive axles ,and
hubs etc. that are removed .The cost of two sets of gears ,bearings etc.
Id say if they have a solid reputation to go for it ,a cheaper job isn't
necessarly better .The 280 for speedo calibration seems steep ,check with your GM dealer to see if the computer can be reprogrammed with the vss
vehical speed sensor calibration for the 4:10 gear ,I think they can do that
as those trucks can be had with that ratio.It has to be done however or the PCM or computer will store a trouble code and illuminate the MIL on the dash
that is the( service engine soon lite) or malfuncion indicator lite as its known.
That will affect the ABS also so that code will also set.
Good luck ,good decision on the gear change