Yes, I've seen this on more than just a 2wd..this is a lot to process though..
Do you really want to even read all of this... sigh..
Ok, A 2wd auto with either original 28's and 3.90 gear doesn't like to go above 65mph for too long. How do i know this, because I had this issue on mine, coupled with overheating. My first thing to do was watch when/how i was using the PWR setting on the ECT switch and when I kept O/D on versus off. Next thing I did was since I wanted to run 31's was switch to 4.56 gears with a speedometer gear reduction unit to correct the speed readout, then I used a transmission cooler plumbed into the return line off of the radiator.
Anyone bother to see if you're even getting proper cooling is the first thing, why..because you may have a plugged cooling setup or possibly be risking blowing your lower radiator because this is where the transmission gets it's initial warmup heat from and cooling later on, though not always the best of cooling.
Did they fill the transmission, put it into neutral then check the fluid level or do it in park? If done in anything but neutral you'll have an overfill condition that will cause overheating.
Did you see if you might have fluid dripping from the bellhousing weephole? *Sign of blowby.
Did you check that FIPG was use for the transmission oil pan or cheap cork? Sign of blowby as the cork won't hold the pressure.
Basically you need to see what is the cause of the overheating, and the transmission oil blowby.
..or do like I did and do a manual transmission swap and be done with it for good. The cost of a rebuilt auto is almost the same to swap to 4wd for me, a "Duh" decision. I get 16.5mph town and 18.5 highway versus 12-13 town and 14-15hwy.
[EDIT] I saw you chimed in in another thread so let me be sure you understand this info here...
NorCalBorn said:
*Again, a mechanical speedo does not have any affect on an automatic transmission. As jerrod re-stated in this thread, the speed sensor is what talks to the ECM and Transmission ECU. Trust me here, unless the right side tailshaft of your truck has an electrical output for your speedometer, it doesn't talk, it spins a cable going up to your speedometer. The speed sensor is a completely different unit in and of itself.
Simply put (*minus the speedometer gear reduction/correction box) this is what a 2wd mechanical speedometer out put looks like..obviously a 4wd will be at a different angle and off of the right upper on a transfer case..this is what you see immediately in this picture on the right side.
*Barely in the pic, over on the left side, is the speed sensor unit, black plastic housing, got it?!