Thanks for all the input from all... I changed the lubricant and inspected everything while I had the cover off. I found a bunch of metal on the fill plug magnet but nothing looked catastrophic inside. The wear on the gears looked normal to me but I did have someone cycle it through reverse & drive while I watched. It seemed to have a lot of play but I couldn't find a reason why.

I'm thinking it might just be general wear, the truck has 165K miles on it and I don't think it has been worked on before. Does anyone have an estimate on the cost to rebuild the differential, changing the parts that normally wear out? Are there any good kits to buy, upgrades to consider, or anything to avoid tearing into. I will be doing this myself but have never really messed with a differential before. I do consider myself a decent mechanic though.

 

Your MegaCab needs 75w-90 with no additive.

Your grabbing was probably because of the viscosity of the fluid has changed overtime and had reduced functionality where heat is a killer.

You have a wormgear style differential where additive does absolutely nothing but cause the issue you described from the old fluid. The part where using it when not needed won't hurt...Not true.

 

It is doing the same thing with either fluid in it and regardless of the cause, I can't do anything about what was done before I bought the truck. I have pretty much decided to do an overhaul and am looking for advice on that now.

 

Well, after researching parts and at one point coming close to spending a lot of money, the problem has gone away. It seems after driving the truck for about 100 miles, mostly around town with a little highway before I was confident, the lubrication change has resolved the problem. The only thing I figure changed is the additive had to get cleaned off all the surfaces and replaced by the new correct type of lube. I'm quite happy to have saved an overhaul with 3 quarts of Royal Purple 75W-90, for now anyway. I really hope it didn't do any permanent damage or shorten the life any. I will report back if things start to decline again. Until then thanks again for the help; it is great to have a forum with so many people that are will to assist!