I have a 1998 4dr 2wd A/T 4.3L Blazer. Currently has 120,000 miles. At 118,000 miles I ran into a transmission problem on the road trip. Got on the highway and was speeding up. When it should have switched to 4th gear, it instead either switched to 2nd gear or neutral (the revs exploded). Couldn't get it to go to 4th gear. Pulled over and grabbed some lunch, got back on the highway, problem was gone. Happened again at 119,000 miles. However, after 2 tries to get into 4th gear, it worked, haven't seen the problem since. Not sure if this may be a mechanical or electrical problems. Haven't changed the tranny fluid since I got the car. Worried changing the fluid/filter may brick my tranny. Looking for any knowledge based on my description to see if it's a simple fix or I'm boned and need to get a new tranny.

Also, I bought this at 80,000 miles. It's now at 120,000 miles. Aside from what's recommend, what could use inspection/changing so I can push to 150,000 miles? I've replaced the water pump, A/C compressor, re-worked all the breaks, replaced the spark plugs, and fuel filter.

Thanks

 

Had a similar problem on a long hot road trip one summer. A repair shop replace both 1-2 and 3-4 solenoids along with new filter and fluid...no more problems on the way back.
Solenoids are not very expensive and easy to get to once the pan and filter are removed.

Do not know for sure if this fixed it or not, just no more problems ever again.

Good luck with yours.