This would have been a reasonable time for Jerry et al. to swing through Wisconsin, having ended the GD June tour in the northeast Nice easy stop to make on the way back home. A very perceptive local account (Branigan 1974) explains that the GD had dropped out of a prospective "July 4th superconcert" in Columbus, OH, with The Band and Eric Clapton, potentially dropping them into the laps of some industrious young Oshkosh promoters, who had been chasing Three Dog Night.
Campus and athletic department officials approved the event, and it was ticketed:
Tickets were going quickly: 15,000 tickets sold over 11 days in Chicago alone, thousands more in other upper midwest towns. Attendance expectations rose to 30,000, and apparently the good burghers of Oshkosh got worried about lots of Dirty Fucking Hippies coming to town. Bratigan (1974) gives some great color. Local homeowners threatened to block an awaiting rezoning petition that would have held up some university athletic facilities -- how that's kicking 'em where it hurts! The local Kiwanas worried about the impact on their annual bratfest and fireworks beerbash. "We don't have enough brat for 30,000 hippies and us too!", said one local.
The university caved. Those town-gown relations can be very delicate. Apparently an alternative venue, the Loft golf course and nightclub in Fitchburg, was selected, agreed, and advertised at some point, but that one, too, was blocked when a local neighbor sued to stop the show. According to (3), "on the eve of the Fourth of July, the promoter [Zoom Productions of Chicago] apparently decided to abandon the whole idea." A column (2) the day of the show says that "Today's Grateful Dead concert ... has been postponed until Saturday [ed: July 6th] at 2 p.m. and relocated at [sic] the Ridgewood Trace Park in Madison." This doesn't seem to have happened, either.
In the end, Bratigan reports that "The Grateful Dead spent four wonderful days and nights in the Pioneer Inn" watching the lightning, victims of "a complicated dance of politics and home-grown fear". We know that Jerry was in NYC playing with Merl on the 1st-2nd-3rd, and I doubt the band was stuck in Oshkosh (though the roadies, quippies, and bag-men might have been). But who knows?
(1) Gold, Aaron. Tower Ticker. Chicago Tribune, June 20, 1974, p. B2.
(2) Gold, Aaron. Tower Ticker. Chicago Tribune, July 4, 1974, p. C2.
(3) Boxoffice, July 22, 1974, p. 49, via issuu.com.
(4) "Grateful Dead Seeks Damages," Billboard, v87 n1, January 4, 1975, p. 3 (via Google Books).
(5) Branigan, Brian. 1974. A Dead 4th of July. Unknown publication (WI).