1994 GMC Van ABS light and engine shifting issues

Good evening!

I have a GMC vandura conversion van. Just after we purchased the van the ABS light in the dash, flashed on and the engine seemed to "pull down" it was pretty quick and I almost thought it didn't really happen.

Well recently while driving home I came to a stop light and slowed to a stop, upon accelerating the dash light once again came on and the engine did in fact pull down, my speedometer wouldn't register over 5 miles per hours. It wasn't as if it slipped out of gear more like a slowing down and creeping along. I pulled over, turned it off, waited a bit, started it up and was able to drive a bit further down the road without issue.

We stopped at a gas station, upon leaving the station the light came on again. I was able to drive but this time it would only get up to about 30 miles per hour, it felt like it wouldn’t shift. So once again we crept along.

We pulled off. My husband unplugged the ABS fuse, everything started fine and we were able to drive home without further issue.

We took the van to a mechanic who said we needed a several transmission parts replaced or a whole new transmission. We were told that the ABS codes were probably just a coincidence. I am not a mechanic but I don't think it's the transmission but that it is related to the ABS light.

We pulled the codes off of the ABS and they were "rear wheel sensor (signal?) Faulty" and "Rear wheel sensor (signal?) Missing". The Check engine light also came on and it referenced the EGR valve.

Should we replace the sensors on both rear wheels? Do you think it is transmission issues (we replaced the trans filter and fluid, it was red, a bit dark but didn't smell burned nor was it brown). Is the EGR valve involved in the above issue?