Couple PMs/emails:
Why 24 inches: Didn't want to risk a narrow track and have the dolley fall over. An axle with drums/wheels is not something you want falling over, even from a few inches. You might get away with using 20 or 18 inches wide... but 24 inches is a proven winner
Will it fit a Banjo: YES. I'll have photo's of the next dolley with banjos and also split axles stacked.
Cheaper casters: The casters I bought are regularly on sale at HF. Right now they are like $6.99 (reg. $7.99) each. These are the highest load rated wheels before you get in the $15-$19+ range. For the $30 these casters cost vs lighter ratings go ahead and try it and let me know... but I like my toes and plan to keep them. Ignore the review from the nit wit on the HF site.
Bolts: Go to TSC and buy the screws by the pound. The ones I used for the 4x4 had 9/16ths head and were 5 inches long. The side supports had 1/2 head and were 3 inches long. IIRC all the bolts were less than $10.
Why not pressure treated wood: I bought what was cheaper. Pressure treated lumber is 'wet' and kinda nasty to work with. I don't plan on putting these outside, but even if I did the regular wood is not going to rot for a long time.
Do you need to take the tires off: Yes. The axle would have to be significantly higher to fit wheels and keep them off the floor. Then there is the risk of the thing tipping over (maybe the 24in of separation would stop it... don't know). My need is to move axles WITHOUT tires.