Both of differential covers are leaking on my truck. Should I replace gaskets or just put RTV in place of a gasket?

 

Myself I would replace the gaskets, on my 06 they can be reused.

 

Use Ultra Black or Ultra Grey. Both work far better than a gasket. But any silicone's ability to seal will only be as good as the surface is clean. Get ALL the oily residue off both surfaces first.

 

also let the silicone partially set up before tightening the bolts

 

X2 on letting the stuff set up for an hour or so, THEN tighten them all the way down and fill with fluid.

 

Make sure you check the directions on whatever you use..... Some silicone specifies no rest time to allow skinning and to install immediately. Also, dont forget about the bolt holes. Sometimes they can reach all the way through to the fluid and if you dont clean and silicone them too, then they can leak.

 

I install the cover immediately, I just don't tighten the bolts down the last turn or so, till I let it dry a bit.
I fill the fluid up after it cures as well.
My logic it it leaves a complete gasket everywhere rather than just tightening it all the way down and "bottoming out" the cover on the axle surface.
I used that tech. on my old '72 mud truck Dana 60 axles which I used to have the cover off at least a couple of times a year, flushing out water and mud.
I figured having a little more "cushion" of gasket would help if - more like when I "tap" my diff cover on a root, log or rut and potentially started a little leak.

 

I use Katooms technique and have zero leakage or seepage over past 5 years.