1999 4x2 149,700 miles auto trans. I have searched this site for hours and can not find anyone with same issues. All at once I had a bad slip in 1st, tc lockup at 20mph and when I switched off o/d all hell broke loose (felt like the tc was quickly locking and unlocking).
1.changed filter and fluid....it was darker than I wanted, filter was ok, but still ran bad
2.checked grounds, checked ele. conections and replaced pressure gov. and solenoid.....The tc problems are gone and it shifts good once I get by 1st, but I still have a slip in 1st. It only happens half the time but dissapears compleatly when o/d is turned off. If I start to feel it slip I can turn off o/d and the truck will almost slam into 1st gear. I can also cure the slip by putting the truck into park or neutral for a few seconds. Any ideas before I take it to a trans. shop. I really think it is electrical as the problem comes and goes, and can be resolved without o/d. I must be overlooking a switch or sensor. Thanks, Tim

 

Think it is time to pull the transmission and look inside.