Strange trans behavior on 96 RMW

A couple weeks ago I set off on a 600 mile trip to school with basically all of my personal belongings and the transmission threw a fit about 50 miles into the trip. I recklessly pressed on at about 63 mph with the engine turning 3600-3700 rpm. Eventually I made it to my apartment 11 cheek-clenching hours later. I checked the 4l60E issues thread and nothing really matched what was happening. While on the highway, if I let off the throttle the engine instantly goes down to 650-700 rpm. Applying throttle from here just revs it up to about 3500 rpm until the trans "catches" and it can put power down. The car was running perfectly the day before the trip, although it did show a p1870 code for about 15 minutes several weeks earlier that went away on its own. According the seller, the transmission was rebuilt in 2014 so if that's true I have a hard time believing it's done for.

Given that I'm at school with basic tools and limited budget, what diagnostic tests can I do to help narrow down this issue?

I'm pretty new to the B Body scene so if anyone around the Columbus, OH area wants to help out that would be much appreciated.

Thanks in advance for your suggestions

How did you know the engine was turning 3600-3700 rpm?

A quick guess is the 3-4 clutch pack has worn out, and now you only have 1-2 gears,.

the 1870 code was to let you know its been driving without lockup on the converter,.

Fluid smell bad?

*edit* 27" tall tires with 2.93 ring and pinion ratio 1.62 trans ratio = 3500 or so at 60 ish mph

little thirsty on fuel i'll bet,.

I have a scangauge plugged into the obd II port. The fluid is at the correct level and looks normal. It does smell a little funky though. It stayed at a consistent 3600 rpm for like 500 miles and didn't increase at all

for what ever reason,. the trans has no 3-4 gears,. i,d guess 3-4 clutch pack from my nice comfy couch here,.

That seems pretty logical. Trying to remember if there's anything else happening. I've done a lot of mechanical work on my Jeep but never anything inside the transmission. Would it worth fixing the clutch pack or should I find a replacement trans?

Hey another Columbus Ohio wagon person! Welcome to Columbus, and the forum?

If the 3-4 clutch pack goes out, I'd expect the typical "1-2-oops no more gears" failure mode. This sounds different. From a standstill, what happens? Does it behave normally until it tries to shift into 3rd?

if you are motivated,. and not susceptible to peer pressure,. then yes pull the trans and repack the 3-4 clutch pack,.
I,m  sure you will hear all kinds of advice,. but its something i have done twice now on a daily driver,.
and if you are careful,. and the transmission is not contaminated with metal filings you can clean the sludge from the pan and the valve body, paying close attention to the force motor screen,and reassemble it, and it might last you quite a while, or it just might last you a week,.
Usually 3-4 clutch packs wearing prematurely has a lot to do with trans tuning,. and with trans pressure (or lack of it) due to plugged filter etc,.
If like me you either have plenty of time and no money,or plenty of money and no time, that usually dictates my course of action,.clutch discs are pretty cheap for 4l60e

youtube is a great learning channel,.

I liked watching this guy's series on 4L60E teardown and rebuild. Makes it look like a piece of cake.