THis is a tuff question to answer accurately because there are a lot
of varibles here. You can drive it for a while but you should consider
down shifting on long hard pulls to easy strain on drive clutch (if
this is the problem) and lower tranny temps a bit. If you nurse it it
may run some time.  If you have oversized tires on it, go back to
stock as this will ease tranny strain noticably. If I did a rebuild, I
would do a complete overhaul because the parts are not the expensive
and the biggest part of overhaul price is labors anf the difference
between a partical and complete rebuild is small.

WHen you look for a long term solution, consider this, back in the
70's and earlier Detriot used to gear truck differently for sales in
mountain states because of thinner air and longer climbs. They stopped
doing it when federal emissions and MPG rating came on board.  "If"
you plan to keep truck for a long time nad you run bigger tires you
would do well to consider regearing the truck as it will reduce tranny
strain. (it would extend life of old one the same as small tires
would). Myself, I am consideing moving to CO for earlier retirement in
about 5 years and when I do I am going to regear my trucks. I have a
stock 89 1500 4x4 burb that I bought new that has been in CO many
times all aver state and up pikes peak a few times too. It has 3.73's
gears with stock tires and it does fine at lower altitudes but above
8000 feet or so it really get weak and OD about worthless.  The burb
is still cherry and if I take it there it will have 4.10's in it or
deeper still if I put bigger tires on it.

On additives, I would change the fluid and filter and keep it fresh
and keep a eye on it. As long as it has the funny kinda sweet smell
you are okay but if it starts to stink or smell like burnt peanut
butter the end is near.  Also if it starts to get brownish red or
darker still it is not a good sign either.
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