OK. So after changing fuel pump and a bunch of other stuff on my 2001 GMC Jimmy 4.3L another problem seemed to have surfaced. Any of you guys out there ever run into this problem:

The car runs just fine when you drive it but when your wife or girlfriend drive it something happens? :) This isn't a shot against the ladies out there but this always seems to happen to me. This time though I witnessed the problem myself.

So, the Jimmy goes into reverse and drive a little slower than normal. It's been doing this for awhile according to my wife. Also when driving it, especially when the engine or outside temp is cold, it will intermittently slip out of drive. It only does this once or twice but after it warms up it doesn't do it. So my question is -- Do you think it might be time for a tranny fluid and screen change? The truck does have 121,000 miles on it and I doubt the tranny has ever been serviced. Any suggestions?

Sounds like the filter might be plug up. Try changing and hope that is all.

Transmission fluid and filter should be changed every 36,000 miles, more often if you tow with the vehicle.

Ok, I checked the transmission fluid. It is not foamy and is at the correct level and doesn't smell burnt. However it is a rather dingy shade of reddish brown so I believe a change is in order. Now for another question. When I change the fluid and filter I'm only going to be removing around 40% or so of the fluid that's there. The remainder will still be in the transmission in the valve body, etc. I'm not up for doing a flush since I've heard bad things can happen when you do that. So here's my question. If I change the fluid now and then wait about a month and change it again eventually I will have changed all the fluid through attrition. Is that the right way to go or should I just stick with this one change and allow some contaminated fluid to remain? Thanks in advance for the replies. Also does anyone know this transmission's fluid capacity? How much should I consider putting back in once I drain the pan?

Try changing it first and see if it shifts better. If it does change again in a month or two. You will still have some bad fluid left in but it should be a lot better. Buy about 5 qt. and add one and check the dip stick with the engine idleing. keep doing that until it is fill. Don't overfill.

You might also want to try Lucas tranny oil addative. I put it in mine and it seemed to help the hard shifting. Says it also helps with slippage. I'm not much up for fix it in a bottle but it seemed to help.

"Fixes hard shifting and slippage", two opposite problems. How does it know which problem to fix? When you pour it in, do you tell it what you want it to fix? Sounds like sales hype BS to me. Chemical engineers formulate a certain "package" of additives for each type of lubricant. I'm not sure how all of those chemicals will react when mixed together, and I'm certainly not going to let my vehicle be the guinnea pig.