Getting someone to access the tank, cut in and install a proper access port, and do a proper cleaning in it is probably going to be worth 6-8 hours of your time, so $400 doesn't seem unreasonable.
The option is to remove your tank (access varies) then cut the access hole yourself, get a port to fit in there, spend some time making sure you get all the metal filings out of the tank, clean it well, dry it back out....You'll probably only be out of pocket $100 for the access plate and whatever cutters and sealants you need, but it will take a while.
"The University of Idaho conducted tests on the life expectancy of fuels to determine the timeline on degradation of stored #2 diesel fuel. The results indicated 26% degradation after 28 days of storage."
Source Of Diesel Fuel Problems
Sounds a bit extreme to me, and ignores the question of crud growing in the tank and contaminants and all. But I do know that pump gasoline is designed to be used within 90 days, and it breaks down signficantly by that point. I wouldn't expect 2 year old diesel to be good, even if it was good enough to burn. My rule of thumb is that if the fuel isn't going to be consumed and replaced in 90 days, whatever kind it is, it needs stabilant and additives to keep it in top shape.
A diesel engine is a nice thing, except for the Jekyll-and-Hyde show it puts on when there's any little problem in the fuel system. With a tank that's been sitting for two years? No matter how you do it, yes, clean it out thoroughly.
or make sure your SeaTow policy is paid up.