1994 Chevy Truck 4 wheel doesnt work very often in rever

Well, after I had wrote the original question, Reverse is totally out on my truck unless its in 4 Lo. I had to use reverse, so I put it into reverse in 2 wheel drive and smoke came out from under the truck and it wouldn't go anywhere in reverse, it would whine like a gear drive timing set as the RPM's would increase. I checked trans fluid and it was full where it should be but had definetly gotten darker in color and smelled burnt. I was told that it will go in reverse in 4 low because the gearing is so much lower but even that will eventually go out. Was also told that I had lost the "reverse/low band" in the trans. Sounds like I will buying a rebuilt transmission on Monday. Let me know if you have any more thoughts. Thanks