Hey guys. I have a white 98 Chevy Tahoe LS 4Door (5.7L gas). So im wanting to replace my rear diff oil and transfer case fluid. What im actually wondering is what your preference of fluids are for older trucks. Believe its limited slip on back. Anyways its probably close to 100xxx' k since its been changed ( same with tranny fluid). So i was going to get lucas trans fluid, but still unsure on what to get for rear oil and transfer case. Unless its better just to leave the old oil in? I cant see howd it be better though. Anyways thanks for any advice or thoughts you may have. Like to even get a idea what you guys run in your daily tahoes!. Thanks again, any suggestions or info is appreciated

First you need to find out what kind of rear end you have. If you have the standard issue limited slip rear end, 80-90W oil is fine. But if you have the highly sought after G80 locking diff, that thing takes a special oil. Take a picture of your RPO sticker(glove box) and post it here. We can find out what you have.
Your transfer case is kinda the same situation. Depending what model you have, will determine what oil it needs.

Right on. Does seem i have the g80 rear as you were saying. I appreciate the help man!

Now I remember, GM transfer cases are fluid sensitive. Some require GM Autotrac fluid, Dex 3 fluid or regular transmission/gear oil. This depends on the type of transfer case you have. Best and safest bet is to call your local GM parts dealer, give them the last 8 of your VIN and they can tell you exactly what you need. Same goes for the rear end.