That there be a 1990-1991 truck with rear wheel ABS, yes.
There are two or three different makes of brake computers, Kelsey Hayes and ... something else. Post up any markings you see on the computer -- it's the black box by the brake booster.
Also, disconnect the battery(ies), wait fifteen minutes and reconnect them... this will reset the black box. (You could also disconnect it, but it's got multiple plugs and is a pain.) I find that the RWABS is mildly b!tchy and does this sometimes ... it's a pretty lame ABS, as they go, and on my truck that had I ended up just disabling it rather than dink with it.
-- A