It sounds like your 3rd gear clutches are shot or your 3rd gear sprag is bad and not holding which will cause your condition and this means rebuild to repair the problem. I cant help you out much on interchange other than you really need to replace the trans with what you took out of it as there are numorous variations of that transmission. Morad Parts Company may be your best bet for a known good trans that will be a correct replacement for your car. The easiest way to distinguish a 4T60E from a 4T65E is is that a 4T60E will have a vacuum modulator canister on the front side of the case that points straight forward, a 4T65E does NOT have this.
