Do what will put you most at ease, but I might suggest that rebuilding/replacing the transmission may be premature if it is performing fine. Prior to our trip out west this summer, I had the same concerns about whether the trans would hold up to the long-distance towing (9-year-old SUV with 130K miles towing a 4500lb HTT). As someone else suggested, I just did some PM to give myself some reassurance for the trip. The PM included:
- Changed all the drive belts
- Changed transmission fluid
- Changed spark plugs
- Installed a trans temp gauge.
- Check tires/brake condition
- Installed new tires on the HTT (old ones were 10 years old anyway)
- Not PM, but joined Good Sam road side assistance just in case.
Once all that was done, took a 2000+ mile round-trip tour to the Black Hills and back with no problems.
