The white plastic things are teflon blocks--if you can get to them to remove then, how come you can't replace them--not understanding, I guess.
Last summer I had to do something similar on my bedroom slide--got the replacement teflon blocks and did the work. Extended the slide partway and placed a tripod adjustable jack under a 4x4 that reached across the entire slide bottom. Carefully jacked the slide upward untill I noted solid resistance--this gave me enough room to work under the slide floor. Would it be possible to do something like this on yours?
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died