transmission binding up??

rbrtwbstr pretty much listed items that I'd agree need very close inspection.

You list 18 yrs exp, so I'm sure when you try to get it into gear, you are also trying letting the clutch engage just enuf to rotate your input shaft (to eliminate a "between gear" non-meshing). BTW, when you do that... any effect or other abnormal signs?

With that "bumper modification", it would be best to post some full views & some close-up pics. Frame rails should never be welded, except under very specific & very limited DOT approved circumstances (I'm not familiar with all specifics), and the fact that this welding at the rear of frame rails (exact position I'm assuming) is a VERY torsional/"high flex" frame area. Removing a factory crossmember is a huge "No-No".
I'm still trying to picture this added "bumper" and also wondering if you have proper clearances to trailer/landing gear when jacked.

The fact that the welds are cracking is very troublesome. This also may be now allowing unwanted/unrestricted frame flex, whereas the original crossmember limited this movement and retained proper structural integrity.

AS 379exhd said, I definitely would not have that shop do any further re-welding. Period! Any welding is either 100% correct the first time, or nothing. Welding cracks may also indicate a welding modification in the WRONG structural place, that will crack regardless of how good a welder is. All welding must co-incide with proper structural location/design.

To me, it seems to be either a "mechanical object" obstruction preventing shift lever movement itself (remove your shift boot to help viewing) OR situation of significant stress misalignment of trans/drivetrain due to effects of frame rail torqueing/misalignment.