2000 GMC 4.8 Trasmission Poppping & Pulling during startup

[cardriver]

2000 GMC Sierra 4.8
260,000+ Miles
Automatic Transmission (not sure of model, stock as far as I know)
Lost 2nd gear in tranny, so local auto shop replaced it with a used one about 1 year ago.

Ever since the tranny was installed I had some weird quirks.. Sometimes when starting the truck, the whole thing would lunge forward as if it was trying to start in gear. It would only happen for a second, and make a popping sound at the same time. This would mostly happen when the truck has been sitting for a while, example: Drive to work around 6am, park it and let it sit in the same place. Start up around lunchtime, and it would do it. Leave it from lunch to quitting time, and it does it. Maybe the popping is my u-joint, something else I need to investigate (I do have a clunking back there when going from park to reverse to drive, etc.).

Just a few weeks ago, I was pulling a small trailer with a couple of 3 wheelers on it, had overdrive locked out and the shifter in 3rd. Had to do this to keep from dropping below 70mph, and had a some small hills to get up and down. I was pushing it a bit too hard here and there and had a "transmission fluid hot" light come on. I slowed down to around 60mph until I could stop. The light went off shortly after it came on. Fluid was on the underbelly of the truck looking to be from the overflow, fluid seemed fine (didnt smell burnt, was still red, not foamy).

I checked my fluid level shortly after that trip again, and it seems to be over full. There are two dots/holes in the dipstick that I "think" is the operating range. It is well past the top one. I think I am hearing a little "rattle" noise sometimes when it shifts from 2nd to 3rd. Now when I start it sometimes, it still lunges, but now lunges in reverse or either I hear a rattle anywhere from 1-2 seconds long, but no lunging.

Where should I start on this thing? I have a bit of a problem with my ignition where I have to jiggle the key just right while turning it when starting the truck. I've got it down pat, but anyone else that tries to start my truck can't. I took it back to the local auto shop to have them check it lunging forward when starting, and they said they couldn't get it to replicate the problem. But when I picked it up, they couldn't even start it to drive it around from where it's parked. So I'm thinking they couldn't even start the truck to check it, and just fed me crap to get me out of there. Any help would be GREATLY APPRECIATED!

[maxwedge]

First fix the ign switch, no? Without checking this out onsite it is difficult to diagnose this. Is it in park and you start it and it lunges? if so the linkage is the first thing to check that it is actually completely in neutral, if ok something is going on in the unit, Also get the fluid level to the proper point, all basic issues.

[cardriver]

I will be looking into getting the ignition fixed here very soon. Yes, it is in park each time I start it and it lunges. It only seems to do it after it has been sitting and not running for a while. How would I go about checking the shift linkage? Am I correct about the fluid level on the dipstick, should it be between the two indicator holes at operating temperature? Thanks for the quick reply max.

[maxwedge]

Yes between the 2 holes, disconnect the linkage at the trans when in neutral, wheels blocked and/or brake on, see if it is adjusted properly in that when the trans is in neutral the linkage fits exactly back in the shift lever without any movement.