TPS would be the very first thing that i look at. all your symptoms including the late shifts points to exactly that. there is a TPS diagnostic chart located in The Reference Thread in the general truck section on how to test this with a halfway good multimeter. if this does turn out to be the problem, clean out the port that the TPS reads out of on the side of the TB as this can develop pretty heavy carbon deposits in there. do not spray the TPS or IAC as this can cause them to fail.
TPS, throttle position sensor, should be loacted on the drivers side of TB
IAC, idle air controlling sensor, should be located on the rear side of TB