The early 99 had emission tuning in it that will not allow full throttle to be obtained until the rpm's get brought up and feedback from the VSS are calculated. You are hard pressed to get a STOCK early99 to spin the tires. Get the exhaust, intake and dp tunes and you will have a whole new truck. I have had mine since near new and the power off the line was not there until I started modding it.
A freind of mine has an 8.1L chebby. She milkes it for all it is worth to get a whole 14mpg on the highway. Local she complains about 11mpg.
Keep the diesel, it will be your friend for a long time.