Summit has been great to me, even returns and exchanges they are great to work with. Jegs, not so much. They are usually always a cent or two higher than Summit, and shipping prices a little more. Those shitbags at Randy's Ring & Pinion if you have a return will **** you hard. They ship from the central part of the country but returns have to go to the west coast, which costed me a bundle. Add on their restocking fee and pretty much **** those guys. Summit can get you the same **** cheaper.
Another driveline parts place I deal with is PartsMike. A lot of the stuff is Jeep related but if you're running tons they have some great upgrade parts.
http://www.partsmike.com/
You can also get some really good deals from Complete Offroad if you're signed up for their emails.
http://completeoffroad.com/
Extreme Axle Sales has some slick conversion hubs, if you want to run your 6 lug wheels on a dana 60 or 14 bolt rear as well as a bunch of other stuff. Ordered complete SRW Dana 60 hub assemblies for the big truck from them.
http://www.extremeaxlesales.com/
Denny's Driveshafts has upgrade parts like 1350 or 1410 yokes, ordered many times from them.
www.dennysdriveshaft.com/
Polyperformace has a good selection of parts as well as heims if you're building links.
http://www.polyperformance.com/
And of course everyone knows Offroad Design. Ordered too much **** from them to list it all here.
http://offroaddesign.com/
As you can see I mostly have links to good sites for driveline parts lol