When we got our NTU coach, we drove it back from SW Texas or about 1500 miles. We never had a DP before so the heavy bumble bee vibration I felt in my throttle foot and seat I thought (hoped?) was normal.
When we got home, I put the DP on blocks and crawled underneath. What I found was that:
1. The air leveling valves (all 3 of them) were leaking AND the ride height was adjusted incorrectly.
2. The drive shaft slip spline and u-joints had little if no grease in them.
I replaced the air leveling valves, adjusted the ride height per Spartan specifications, dropped the driveshaft, cleaned the splines and regreased them and replaced the u-joints with new HD ones and regreased them as well.
The vibration was instantly gone and the coach rode 500% better (1000% better after I installed the Koni FSD shocks).
If you can't safely check these items out, have a qualified shop check them out.
FWIW, the original u-joints (and shaft) are Spicer and are about $150 each
...so I went with NEAPCO which are identical in quality and are 1/3 the cost
(and made in the good ole USofA).