You'll need to pull the codes to get a start on this problem. A 99 will need
a code reader or scan tool of some sort. If you JUST bought it, my
suggestion would be to check with the dealer. If a private sale, I'd ask the
previous owner to give any info he could, or at least help with the repairs
in some way.
Once you pull the codes, disconnect the battery and allow time for the PCM
to drop voltage. Reconnect the battery and drive the truck, and check for
codes again. This should point directly at your problem if its electronic.
If you cannot confirm that a code is set or repeats, but the truck still
malfunctions, pull the pan and look for debris. You don't say if it slips
while in gear, but if its hydraulic/mechanical, it could be either the rear
clutch in the main trans or the direct clutch in the OD section. Depending
on previous use and maintenance, 103k might be the rebuild point on this
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