I have a Titan 2WD KC Titan that upgraded to Tru-TRac & Mag-Hytec cover and I found that exactly 4 quarts to be the optimum volume that brings the level to about 1/8" below the lower fill mark on the Mag-Hytec dipstick.
Many other who have exceeded the 4 quart volume were getting oil blow-thru on the vent mod.
There is a good read about the lubricant fill volume for the Mag-Hytec cover here: titanspot . com/Titan/index.php?showtopic=5892
I would have posted this message and the link on your thread about Mag-Hytec diff oil volume but TT won't allow a link to TitanSpot so I hope the link comes through on the message. You will need to remove the spaces between titan spot . com