What causes the truck to jump out of gear? on 1995 Chevrolet Tahoe

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This is a 4x4 tahoe. Hi and low range work. The transmission shifts great, just sometimes it jumps out of gear while driving expecially on inclines. exa. pulling up on ramps to change oil. What could be the problem?
(1) Answer
if the fluid is full, it is internal.
if you are talking about the transfer case, it could be a faulty syncro or uneven tires.