Ok, I have a 91 Chevy ex-cab Z71 4x4 that came off the factory with a TBI 350 and 4-spd auto tranny with lockup T/C. However, I'm having a hard time idenifying the tranny because I don't know much about Chevys. Is it a 4L60-E? TH350? 700R4? The only idenification number I can find on the tranny is 15522125 and that's stamped on a plate next to the upper corner of the oil pan.

The reason why I'm asking is that it's not shifting right. It holds a gear much longer than it should. Like it'll shift into 2nd gear at 3000 rpms and into 3rd at 2600 rpm and the torque converter doesn't lock up anymore. Other than that, it shifts smooth and all the gears don't slip...any ideas?

I'm not real familiar with the "new" body style GM trucks.
I did own a '95 Z-71 once.
I do know these trucks, when they first came out in '88, had the TH700R4 tranny's.
But I do not know what year the 4L60E came out in. :(
BTW, my '95 Z had this 4L60E tranny.
You can rule out the TH350, as those were not available in stock form since way back to '82 or '83.
The TH350's you have probably seen in '84 and newer trucks were swapped in.

A 91 chevy with a 4sp auto is a 700r4 the 4L60 didn't come out till 93 and its basically the same as the 700r4 only its electrically controlled

I have a stock '92 3/4 ton 4x4 'light duty' so named because it is the six lug 3/4 ton with the 9.5 inch ring gear in the rear end. It came stock with the 4L60-E. So some of the 92's had the 4L60-E, I have never heard of a '91 having a 4L60-E. I think '91 was the last year of the 700r4/4L60.

Alright, so I'm gonna go ahead and say it's a 700R4. Dude at the junkyard said it's a 700R4 too. :) Will the 700R4s out of the Camaros and 2WD trucks fit?

And any ideas on why it holds a gear longer than it should? Is it just messed up? :D

IIRC .. yes it will fit but the output will need to be swapped from your tranny to the new one. it would be better to find a 4wd version of the tranny unless you plan on having someone go thru it anyway. the car and 2wd trannys were thinner cased than the 4wd versions. so far as the shifting late is concerned it could be the TV cable needing adjusting.